[CHORE] Remove github.com/yuin/goldmark-meta

- Remove a unused dependency. This dependency was added to handle YAML
'frontmatter' meta, parsing them and converting them to a table or
details in the resulting HTML. As can be read in the issue that reported
the behavior of YAML frontmatter being rendered literally,
https://github.com/go-gitea/gitea/issues/5377.
- It's an unused dependency as the codebase since then moved on to do this YAML
parsing and rendering on their own, this was implemented in
812cfd0ad9.
- Adds unit tests that was related to this functionality, to proof the
codebase already handles this and to prevent regressions.

modules/markup/renderer.go

@@ -78,7 +78,6 @@ type RenderContext struct {
 	ShaExistCache    map[string]bool
 	cancelFn         func()
 	SidebarTocNode   ast.Node
-	RenderMetaAs     RenderMetaMode
 	InStandalonePage bool // used by external render. the router "/org/repo/render/..." will output the rendered content in a standalone page
 }