feat(highlights)!: enforce documented captures (#6232)

Problem: Allowing undocumented "secret" (sub)captures makes it harder
to write comprehensive colorschemes and catch inconsistent captures.

Solution: Only allow captures listed in CONTRIBUTING.md. Add useful
(cross-language) subcaptures and drop language-specific or too niche
ones.

Follow-up: Adding further `*.builtin` captures and changing queries to
use them.

Language-specific subcaptures should instead be added in user config or
a custom language plugin.

queries/markdown_inline/highlights.scm

@@ -16,10 +16,10 @@
 ] @string.escape
 
 ; Conceal codeblock and text style markers
-((code_span_delimiter) @markup.raw.delimiter
-  (#set! conceal ""))
-
-((emphasis_delimiter) @conceal
+([
+  (code_span_delimiter)
+  (emphasis_delimiter)
+] @conceal
   (#set! conceal ""))
 
 ; Conceal inline links