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/markdown.go

@@ -13,6 +13,7 @@ import (
 	"code.gitea.io/gitea/modules/log"
 	"code.gitea.io/gitea/modules/markup"
 	"code.gitea.io/gitea/modules/markup/common"
+	"code.gitea.io/gitea/modules/markup/markdown/callout"
 	"code.gitea.io/gitea/modules/markup/markdown/math"
 	"code.gitea.io/gitea/modules/setting"
 	giteautil "code.gitea.io/gitea/modules/util"
@@ -124,6 +125,7 @@ func SpecializedMarkdown() goldmark.Markdown {
 				parser.WithAttribute(),
 				parser.WithAutoHeadingID(),
 				parser.WithASTTransformers(
+					util.Prioritized(&callout.GitHubCalloutTransformer{}, 9000),
 					util.Prioritized(&ASTTransformer{}, 10000),
 				),
 			),
@@ -135,6 +137,7 @@ func SpecializedMarkdown() goldmark.Markdown {
 		// Override the original Tasklist renderer!
 		specMarkdown.Renderer().AddOptions(
 			renderer.WithNodeRenderers(
+				util.Prioritized(callout.NewGitHubCalloutHTMLRenderer(), 10),
 				util.Prioritized(NewHTMLRenderer(), 10),
 			),
 		)