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/outline.scm

@@ -0,0 +1,9 @@
+; Show each cart section as an outline entry, named by its marker.
+(lua_section (lua_marker) @name) @item
+(gfx_section (gfx_marker) @name) @item
+(gff_section (gff_marker) @name) @item
+(label_section (label_marker) @name) @item
+(map_section (map_marker) @name) @item
+(sfx_section (sfx_marker) @name) @item
+(music_section (music_marker) @name) @item
+(unknown_section (section_marker) @name) @item