Allocate tree-sitter's external scanners from the same Zig allocator as its runtime

syntax.zig points the tree-sitter runtime at a Zig allocator, but the
grammars' external scanners did not follow it. A grammar's
tree_sitter/alloc.h maps ts_malloc to the runtime's current allocator only
when TREE_SITTER_REUSE_ALLOCATOR is defined, and plain malloc otherwise, so
every scanner's state and its arrays came from libc. Five scanners (bash,
markdown, markdown_inline, python, typst) also call malloc, calloc, realloc
and free by name.

build.zig now compiles every grammar with TREE_SITTER_REUSE_ALLOCATOR and
forces in src/tree_sitter_heap.h, which includes stdlib.h and then defines
the four names as calls through ts_current_malloc and friends. A header
forced in from the build rather than a patch to the grammars keeps zig-pkg
pristine. No grammar archive refers to libc's allocator any more; only the
runtime does, for its defaults. A scanner frees only what it allocated
itself, and the syntax tests that create and destroy a parser for every
grammar under a leak-checking allocator pass.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

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