fix(opentelemetry-context-async-hooks): Add exports field in package.json

[DaPotatoMan]

Which problem is this PR solving?

This PR adds an exports definition (default and types) to ensure the package resolves
correctly. Otherwise it fails with the following message:

Did you mean to import "@opentelemetry/context-async-hooks/build/src/index.js"?

• Added an exports field to @opentelemetry/context-async-hooks/package.json
  • default entry pointing to ./build/src/index.js
  • types entry pointing to ./build/src/index.d.ts

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

• Test suite

Checklist:

• Followed the style guidelines of this project
• Unit tests have been added
• Documentation has been updated

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: DaPotatoMan / name: Fahad (531fa6d)