Server-side syntax highlighting for all code (#12047)

* Server-side syntax hilighting for all code

This PR does a few things:

* Remove all traces of highlight.js
* Use chroma library to provide fast syntax hilighting directly on the server
* Provide syntax hilighting for diffs
* Re-style both unified and split diffs views
* Add custom syntax hilighting styling for both regular and arc-green

Fixes #7729
Fixes #10157
Fixes #11825
Fixes #7728
Fixes #3872
Fixes #3682

And perhaps gets closer to #9553

* fix line marker

* fix repo search

* Fix single line select

* properly load settings

* npm uninstall highlight.js

* review suggestion

* code review

* forgot to call function

* fix test

* Apply suggestions from code review

suggestions from @silverwind thanks

Co-authored-by: silverwind <me@silverwind.io>

* code review

* copy/paste error

* Use const for highlight size limit

* Update web_src/less/_repository.less

Co-authored-by: Lauris BH <lauris@nix.lv>

* update size limit to 1MB and other styling tweaks

* fix highlighting for certain diff sections

* fix test

* add worker back as suggested

Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Lauris BH <lauris@nix.lv>

vendor/github.com/alecthomas/chroma/lexers/internal/api.go

@@ -0,0 +1,161 @@
+// Package internal contains common API functions and structures shared between lexer packages.
+package internal
+
+import (
+	"path/filepath"
+	"sort"
+	"strings"
+
+	"github.com/danwakefield/fnmatch"
+
+	"github.com/alecthomas/chroma"
+)
+
+// Registry of Lexers.
+var Registry = struct {
+	Lexers  chroma.Lexers
+	byName  map[string]chroma.Lexer
+	byAlias map[string]chroma.Lexer
+}{
+	byName:  map[string]chroma.Lexer{},
+	byAlias: map[string]chroma.Lexer{},
+}
+
+// Names of all lexers, optionally including aliases.
+func Names(withAliases bool) []string {
+	out := []string{}
+	for _, lexer := range Registry.Lexers {
+		config := lexer.Config()
+		out = append(out, config.Name)
+		if withAliases {
+			out = append(out, config.Aliases...)
+		}
+	}
+	sort.Strings(out)
+	return out
+}
+
+// Get a Lexer by name, alias or file extension.
+func Get(name string) chroma.Lexer {
+	if lexer := Registry.byName[name]; lexer != nil {
+		return lexer
+	}
+	if lexer := Registry.byAlias[name]; lexer != nil {
+		return lexer
+	}
+	if lexer := Registry.byName[strings.ToLower(name)]; lexer != nil {
+		return lexer
+	}
+	if lexer := Registry.byAlias[strings.ToLower(name)]; lexer != nil {
+		return lexer
+	}
+
+	candidates := chroma.PrioritisedLexers{}
+	// Try file extension.
+	if lexer := Match("filename." + name); lexer != nil {
+		candidates = append(candidates, lexer)
+	}
+	// Try exact filename.
+	if lexer := Match(name); lexer != nil {
+		candidates = append(candidates, lexer)
+	}
+	if len(candidates) == 0 {
+		return nil
+	}
+	sort.Sort(candidates)
+	return candidates[0]
+}
+
+// MatchMimeType attempts to find a lexer for the given MIME type.
+func MatchMimeType(mimeType string) chroma.Lexer {
+	matched := chroma.PrioritisedLexers{}
+	for _, l := range Registry.Lexers {
+		for _, lmt := range l.Config().MimeTypes {
+			if mimeType == lmt {
+				matched = append(matched, l)
+			}
+		}
+	}
+	if len(matched) != 0 {
+		sort.Sort(matched)
+		return matched[0]
+	}
+	return nil
+}
+
+// Match returns the first lexer matching filename.
+func Match(filename string) chroma.Lexer {
+	filename = filepath.Base(filename)
+	matched := chroma.PrioritisedLexers{}
+	// First, try primary filename matches.
+	for _, lexer := range Registry.Lexers {
+		config := lexer.Config()
+		for _, glob := range config.Filenames {
+			if fnmatch.Match(glob, filename, 0) {
+				matched = append(matched, lexer)
+			}
+		}
+	}
+	if len(matched) > 0 {
+		sort.Sort(matched)
+		return matched[0]
+	}
+	matched = nil
+	// Next, try filename aliases.
+	for _, lexer := range Registry.Lexers {
+		config := lexer.Config()
+		for _, glob := range config.AliasFilenames {
+			if fnmatch.Match(glob, filename, 0) {
+				matched = append(matched, lexer)
+			}
+		}
+	}
+	if len(matched) > 0 {
+		sort.Sort(matched)
+		return matched[0]
+	}
+	return nil
+}
+
+// Analyse text content and return the "best" lexer..
+func Analyse(text string) chroma.Lexer {
+	var picked chroma.Lexer
+	highest := float32(0.0)
+	for _, lexer := range Registry.Lexers {
+		if analyser, ok := lexer.(chroma.Analyser); ok {
+			weight := analyser.AnalyseText(text)
+			if weight > highest {
+				picked = lexer
+				highest = weight
+			}
+		}
+	}
+	return picked
+}
+
+// Register a Lexer with the global registry.
+func Register(lexer chroma.Lexer) chroma.Lexer {
+	config := lexer.Config()
+	Registry.byName[config.Name] = lexer
+	Registry.byName[strings.ToLower(config.Name)] = lexer
+	for _, alias := range config.Aliases {
+		Registry.byAlias[alias] = lexer
+		Registry.byAlias[strings.ToLower(alias)] = lexer
+	}
+	Registry.Lexers = append(Registry.Lexers, lexer)
+	return lexer
+}
+
+// Used for the fallback lexer as well as the explicit plaintext lexer
+var PlaintextRules = chroma.Rules{
+	"root": []chroma.Rule{
+		{`.+`, chroma.Text, nil},
+		{`\n`, chroma.Text, nil},
+	},
+}
+
+// Fallback lexer if no other is found.
+var Fallback chroma.Lexer = chroma.MustNewLexer(&chroma.Config{
+	Name:      "fallback",
+	Filenames: []string{"*"},
+}, PlaintextRules)