fix(build/js): Add subpath export shims for legacy bundlers

[Lms24]

Currently, adding @sentry/conventions to code bases using legacy tooling (old TS versions or, as we found out, webpack 4), results in build failures. These old tools don't support the exports conditions field or the subpath exports inside of them. Therefore, they'd resolve an @sentry/conventions/attributes import to the file @sentry/coventions/attributes.js rather than the entry point declared in exports.

This PR adds the shim file so that the import resolves to the shim file, which in turn re-exports from the corresponding subpath export entry point file. We're pointing to CJS on purpose here, given the age of said legacy tools.

Why do we care? We need this for using the conventions package in our JS SDKs. We have a webpack 4 e2e test to ensure our SDK packages stay compatible with legacy setups, which failed after adding and importing from the current sentry conventions package. (getsentry/sentry-javascript#21855)

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Semver Impact of This PR

🟢 Patch (bug fixes)

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Bug Fixes 🐛

• (build/js) Add subpath export shims for legacy bundlers by Lms24 in #459

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