Refactor transformChunk to use AST

[joarfish]

This PR refactors transformChunk so it is uses SWC's parser to collect all strings in the chunk's code that contain the base string that is supposed to be replaced with e.g. window.__dynamic_base__ (or whatever the user configures).

Some Notes and clarifications

1. SWC's parse returns an AST where all elements have spans. These, however, do not refer to character indices in the original code, but to byte positions. That is why we need to encode the chunk's code-string into a Uint8Array. To handle this conveniently, the StringAsBytes class is introduced.
2. If the chunk's code contains invalid javascript code the plugin will fail with an exception. Not sure if this case can actually occurs, though, given that the chunk code should, of course, be valid js.
3. The parser's target is explicitly set to esnext to be as inclusive as possible. We could think about ways to translate the vite target to the parser target. This is not straightforward, because although the parser accepts esXXX strings as target, allows for more complex ways to define the target.

[chenxch]

LGTM.

Thank you for making it more robust.