For easier maintenance and completeness, builtins should use the wgsl-spec package

The wgsl-spec crate allows introspection on the WGSL specification, freeing wgsl-rs from having to manually track built-in functions. Of course to achieve the goal of allowing the rust version of functions to run, a manual rust implementation of missing builtins is needed, however the use of the crate will allow flagging missing functions in the CI pipeline.