Unsupported Tree-sitter 0.26 Wasm build

Published versions of the tree-sitter Rust crate through 0.26 compile stdio.c, stdlib.c, and string.c from the directory advertised by tree-sitter-language's wasm-src build-script metadata when targeting wasm32-unknown-unknown.

That integration used a separate, non-thread-safe allocator and an incomplete libc implementation. Current versions of Tree-sitter instead provide their own Wasm libc implementation and forward C allocation functions to Rust's application-selected global allocator.

The translation units in this directory produce an actionable compilation error for old Tree-sitter versions. They can be removed in a breaking tree-sitter-language release that those older runtimes cannot select.