Change package type to module (ESM)

[aduth]

🛠 Summary of changes

Changes package to use ES Modules as the default module type.

This avoids the need for using .mjs extensions, and may eventually enable future revisions avoiding a build step altogether, shipping the original source files without Babel preprocessing.

As mentioned in the included CHANGELOG, this should not have an impact on downstream projects, possible thanks to support for package.json exports allowing a CommonJS to resolve the require-based distribution.

📜 Testing Plan

Verify tests pass:

make test
make lint

Observe that built output via npm run build:pkg includes expected require or import semantics in build/cjs and build/esm files respectively.