RFC split bundler code into multiple submodules

[Charlie-XIAO]

This is a subtask of #66 to avoid crowding a single PR. Essentially, this:

• replaces bundler.rs with bundler/mod.rs which is equivalent
• moves custom AST transforms in bundler/mod.rs to bundler/transforms.rs
• moves everything else except the pub(crate) bundle function in bundler/mod.rs to bundler/common.rs
• decompose three subroutine functions that will be reused in ENH support external dependencies in widget development #66:
  • bundle_into_raw_module bundles entry point into a raw AST module without transforms
  • apply_basic_transforms applies TypeScript and JSX transforms
  • emit_module_to_buf emits a module to a writable buffer

This is just a pure refactoring of code and functionalities should not change. In particular, none of the unit tests for the bundler is modified so if we are passing in CI we are likely good to go :)

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[Charlie-XIAO]

CI is passing, let's see how this fits with #66.