fix: add support for non-bundler module resolution (tsconfig)

[pieh]

We rely on export map actually being used with helper functions exported from this package, but this only works with bundler module resolution ( https://www.typescriptlang.org/docs/handbook/modules/theory.html#module-resolution )

So this:

• adds top level module for those helpers (that re-export from actual implementation)
• adjust code to use extensions which is more standardized and more likely to work regardless of settings (while keeping previously existing entries in export map, to not break setups for people using extension-less imports)
• adds initially failing test with non-bundler module resolution that should prove the fix is sufficient

Fixes #269
Fixes https://linear.app/netlify/issue/FRB-1661/netlifyangular-runtime-is-not-exposing-the-api-it-says-in-the

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