Make rollup-plugin-typegpu work with aliased or missing tgpu object.

[iwoplaza]

Ideas:

• inject ${alias}.__assignAst instead of tgpu.__assignAst
• If an alias to tgpu is missing, we could either: (to be discussed)
  • Add an import at the beginning of the file.
    • (+) More liberal.
  • Require every TGSL snippet to be in a file with a typegpu import.
    • (+) More explicit, we do not add implicit imports.

[reczkok]

I think we shouldn't implicitly add imports. We can have a descriptive error message when it's missing. This should be a very rare situation anyway.