fix(esm): append .js extensions to relative and subpath imports

[harshitanand]

What

Added a post-processor script to append .js extensions to ESM builds for @bitgo/abstract-utxo and @bitgo/utxo-core to fix module resolution errors.

How

Created a lightweight Node.js script scripts/fix-esm.js that finds import/export statements in the emitted dist/esm files and intelligently adds .js extensions to relative paths and specific bip174 subpaths without breaking existing CJS builds.

Testing

• Existing tests pass
• yarn build:esm — CLEAN
• yarn test — 1942 passed, 0 failed

Related

Closes #7660

[kaustubhbitgo]

From the original issue I would suggest to either use tsc-alias or bundler option as they cover multiple edge cases when it comes to resolve imports / other issues in esm.

[harshitanand]

Thanks for the review! Understood — the custom post-processor is brittle compared to tsc-alias which handles the full range of ESM path-rewriting edge cases (barrel re-exports, conditional imports, declaration files, etc.).

Could you clarify which approach you'd prefer me to implement?

1. tsc-alias — add as a dev dependency, run tsc-alias after tsc in the build:esm step (e.g. "build:esm": "tsc -p tsconfig.esm.json && tsc-alias -p tsconfig.esm.json")
2. Bundler option — switch the ESM output to use a bundler (esbuild/rollup) that handles module resolution natively

Happy to implement either. tsc-alias is a minimal change with no bundler adoption overhead — I'll go that route unless you have a preference for the bundler approach.

[alextse-bg]

flushing, please reping when the fix is ready