chore(instr-undici): add ESM output

[david-luna]

Which problem is this PR solving?

This is a different approach of #2954 which uses the current toolchain to generate the ESM files. Basically is another try on a similar PR targeting express which at that time did not have the best tools to work.

Short description of the changes

• set "type": "module" in package.json
• add exports property in package.json
• add tsconfig.esm.json for ESM compilation
• modify tsconfig.json for CJS compilation
• add the .js extension in import statements

2025-07-24 UPDATE: to simplify integration with mocha and ts-node/esm loader for tests I've changed the compilation step

• the default compilation output is ESM
• there is a specific tsconfig.cjs.json file for CJS output
• for now the ts-node/esm loader is set in the test script. IF this change is applied to all other packages we could promote this config to .mocharc.yml

Notice

This PR is meant to be reviewed along with #2954 which does the same but using a new tool for compilation. The intention is to have a conversation on the path we want to take about tooling. Should we stick to the current tools (for compilation) or change them?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Please upload report for BASE (main@215c2b5). Learn more about missing BASE report.

Additional details and impacted files

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  Misses          ?       12           
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[JamieDanielson]

• Just for easy finding, linking to tracking issue in core Publish ESM packages following ESM spec opentelemetry-js#4898