Move mention macro into a goldmark-parser/renderer

testdata/mention.md

@@ -0,0 +1,15 @@
+Hello @{username}!
+Unclosed @{mention
+Not a mention @{}
+Another one @{name}
+Multiple @{one} and @{two}
+In a link [mention @{me}](http://example.com)
+In a code `@{username}`
+In a block:
+```
+@{username}
+```
+
+Indented:
+
+    @{username}