modules/markup: Lift out the GitHub callout transformer

This lifts out the GitHub callout transformer from
`modules/markup/markdown/goldmark.go` to `callout/github.go`.

While there, clean up the transformer code:

- Use a map to look up supported callout types, rather than a regexp.
- Allow the callout type to be in any case, rather than just uppercase.
- Simplified `.Segment.Value()` to `.Text()`.

Signed-off-by: Gergely Nagy <forgejo@gergo.csillger.hu>

modules/markup/markdown/ast.go

@@ -181,32 +181,3 @@ func IsColorPreview(node ast.Node) bool {
 	_, ok := node.(*ColorPreview)
 	return ok
 }
-
-// Attention is an inline for an attention
-type Attention struct {
-	ast.BaseInline
-	AttentionType string
-}
-
-// Dump implements Node.Dump.
-func (n *Attention) Dump(source []byte, level int) {
-	m := map[string]string{}
-	m["AttentionType"] = n.AttentionType
-	ast.DumpHelper(n, source, level, m, nil)
-}
-
-// KindAttention is the NodeKind for Attention
-var KindAttention = ast.NewNodeKind("Attention")
-
-// Kind implements Node.Kind.
-func (n *Attention) Kind() ast.NodeKind {
-	return KindAttention
-}
-
-// NewAttention returns a new Attention node.
-func NewAttention(attentionType string) *Attention {
-	return &Attention{
-		BaseInline:    ast.BaseInline{},
-		AttentionType: attentionType,
-	}
-}