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>

examples/hello.p8

@@ -0,0 +1,22 @@
+pico-8 cartridge // http://www.pico-8.com
+version 42
+__lua__
+-- hello cartridge
+function _init()
+ cls()
+end
+
+function _draw()
+ cls(1)
+ print("hello pico-8", 30, 60, 7)
+end
+__gfx__
+00000000111111110000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
+__gff__
+00000000000000000000000000000000
+__map__
+0000000000000000
+__sfx__
+010100000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
+__music__
+00 41424344