Change shader parser to use AST or WGSL directly

[guusw]

Various options exist

Directly using source AST

In contrast to the current system that translates shards (after compose) into WGSL source code. We can use the new shards lang (shs) ast directly to build shader sources or even directly generate naga modules.

Directly embed WGSL as shader source

WGSL code could be embedded directly into shader block and then injected into the generated shader.