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/injections.scm

@@ -0,0 +1,11 @@
+; Inject Lua syntax highlighting into the __lua__ section.
+;
+; NOTE: This injects Zed's built-in Lua grammar, which does not
+; understand Pico-8 dialect extensions ( ?, +=, !=, single-line
+; `if (cond) stmt`, binary literals, memory peek prefix operators,
+; etc. ). Code that uses those forms will produce parse errors
+; locally, with degraded highlighting in those regions only — the
+; rest of the file will still render correctly. A future Pico-8
+; Lua grammar fork will replace this; see README for status.
+((lua_content) @injection.content
+ (#set! injection.language "lua"))