Preserve BOM in web editor (#28935) (#28959)

Backport #28935 by @silverwind

The `ToUTF8*` functions were stripping BOM, while BOM is actually valid
in UTF8, so the stripping must be optional depending on use case. This
does:

- Add a options struct to all `ToUTF8*` functions, that by default will
strip BOM to preserve existing behaviour
- Remove `ToUTF8` function, it was dead code
- Rename `ToUTF8WithErr` to `ToUTF8`
- Preserve BOM in Monaco Editor
- Remove a unnecessary newline in the textarea value. Browsers did
ignore it, it seems but it's better not to rely on this behaviour.

Fixes: https://github.com/go-gitea/gitea/issues/28743
Related: https://github.com/go-gitea/gitea/issues/6716 which seems to
have once introduced a mechanism that strips and re-adds the BOM, but
from what I can tell, this mechanism was removed at some point after
that PR.

Co-authored-by: silverwind <me@silverwind.io>
(cherry picked from commit b8e6cffd31)

routers/web/repo/commit.go

@@ -400,7 +400,7 @@ func Diff(ctx *context.Context) {
 			Metas:   ctx.Repo.Repository.ComposeMetas(),
 			GitRepo: ctx.Repo.GitRepo,
 			Ctx:     ctx,
-		}, template.HTMLEscapeString(string(charset.ToUTF8WithFallback(note.Message))))
+		}, template.HTMLEscapeString(string(charset.ToUTF8WithFallback(note.Message, charset.ConvertOpts{}))))
 		if err != nil {
 			ctx.ServerError("RenderCommitMessage", err)
 			return