refactor(codegen): use `Uint8Array` for base64 lookup table (#25913)

## Summary

Replace the `Buffer`-backed base64 character-code table with a `Uint8Array`.

`BASE64_CODES` is only used as a lookup table mapping VLQ digits from `0–63` to ASCII byte values. It does not require any `Buffer`-specific APIs, and `Uint8Array` provides the same indexed byte semantics in both Node.js and browsers.

This reduces unnecessary Node-specific usage and is a small step toward making `oxc-codegen` browser-compatible. The hot encoding loop remains unchanged; only the one-time lookup-table initialization differs.

This does not remove the remaining `Buffer` dependency used for the source-map output buffer.