Add v0.1 scaffold: tree-sitter-p8-cart grammar and Zed extension

The grammar parses the .p8 cartridge container ( header, version, and
the named __lua__/__gfx__/__gff__/__label__/__map__/__sfx__/__music__
sections, plus a fallback unknown_section ). The Zed language definition
hands the __lua__ body to Zed's built-in Lua via injections.scm, so
non-dialect Lua code highlights correctly today; PICO-8-specific syntax
(?, +=, !=, single-line if, etc.) will fall back to error highlighting
in those regions only — see README roadmap for the dialect grammar fork.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

languages/pico-8-cart/highlights.scm

@@ -0,0 +1,13 @@
+; The cartridge magic header and version line are metadata, not content.
+(header) @comment.doc
+(version) @comment.doc
+
+; Section markers ( __lua__, __gfx__, etc. ) are structural tokens.
+(lua_marker) @keyword
+(gfx_marker) @keyword
+(gff_marker) @keyword
+(label_marker) @keyword
+(map_marker) @keyword
+(sfx_marker) @keyword
+(music_marker) @keyword
+(section_marker) @keyword