fix: enable multi-line math equations in wiki (#8424)

- When math equation support was added into Gitea it allowed for math equations to be typed over multiple lines via `\[ ... \]` and `$$ ... $$`. Specifically the former delimiters caused problems with legitimate markdown input to be seen as a math equation and therefore was disabled in e1a82a15d3.
- Enable this multi-line parsing for wiki as it's less likely to cause issues in the context of the wiki.
- It is hard to fix this issue in a proper way without investing a good amount of time in Goldmark as explained in https://codeberg.org/forgejo/forgejo/issues/6902#issuecomment-2845317
- Added unit test.
- Resolves forgejo/forgejo#6902

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/8424
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Co-authored-by: Gusted <postmaster@gusted.xyz>
Co-committed-by: Gusted <postmaster@gusted.xyz>

modules/markup/markdown/util/text.go

@@ -7,6 +7,7 @@ import (
 	"bytes"
 
 	"github.com/yuin/goldmark/ast"
+	"github.com/yuin/goldmark/parser"
 )
 
 func textOfChildren(n ast.Node, src []byte, b *bytes.Buffer) {
@@ -24,3 +25,8 @@ func Text(n ast.Node, src []byte) []byte {
 	textOfChildren(n, src, &b)
 	return b.Bytes()
 }
+
+var (
+	RenderContextKey = parser.NewContextKey()
+	RenderConfigKey  = parser.NewContextKey()
+)