Editor preview support for external renderers (#23333)

Remove `[repository.editor] PREVIEWABLE_FILE_MODES` setting that seemed
like it was intended to support this but did not work. Instead, whenever
viewing a file shows a preview, also have a Preview tab in the file
editor.

Add new `/markup` web and API endpoints with `comment`, `gfm`,
`markdown` and new `file` mode that uses a file path to determine the
renderer.

Remove `/markdown` web endpoint but keep the API for backwards and
GitHub compatibility.

## ⚠️ BREAKING ⚠️

The `[repository.editor] PREVIEWABLE_FILE_MODES` setting was removed.
This setting served no practical purpose and was not working correctly.
Instead a preview tab is always shown in the file editor when supported.

---------

Co-authored-by: zeripath <art27@cantab.net>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

1 files changed, 13 insertions, 0 deletions

diff --git a/modules/markup/renderer.go b/modules/markup/renderer.go
index df2c9ebfc6..e934aed925 100644
--- a/modules/markup/renderer.go
+++ b/modules/markup/renderer.go
@@ -283,6 +283,11 @@ type ErrUnsupportedRenderExtension struct {
 	Extension string
 }
 
+func IsErrUnsupportedRenderExtension(err error) bool {
+	_, ok := err.(ErrUnsupportedRenderExtension)
+	return ok
+}
+
 func (err ErrUnsupportedRenderExtension) Error() string {
 	return fmt.Sprintf("Unsupported render extension: %s", err.Extension)
 }
@@ -317,3 +322,11 @@ func IsMarkupFile(name, markup string) bool {
 	}
 	return false
 }
+
+func PreviewableExtensions() []string {
+	extensions := make([]string, 0, len(extRenderers))
+	for extension := range extRenderers {
+		extensions = append(extensions, extension)
+	}
+	return extensions
+}