Add JavaScript media type shortcut

[vibhor-aggr]

Summary

• make js and .js match standard and legacy JavaScript media types
• preserve generic extension normalization and explicit media-type matching
• document the exception and add request/direct API coverage

Validation

• npx --no-install vitest run src/index.spec.ts -t "JavaScript"
• npm test
• npm run build
• built artifact smoke test
• npx --no-install prettier --check README.md src/index.ts src/index.spec.ts
• git diff --check

Fixes #15.

[kilisamemarisaaa]

I reviewed the source change at exact head 6133c48. To verify it without changing the PR, I applied the PR's src/index.ts diff to an isolated worktree at the PR base and ran:\n\n-
pm test: 47 passing (100% statements/functions/lines, 95.91% branches)\n-
pm run build\n-
px --no-install prettier --check README.md src/index.ts src/index.spec.ts\n- git diff --check\n\nA built-artifact smoke test confirms is() and
equest() return js/.js for �pplication/javascript, �pplication/x-javascript, and ext/javascript, reject �pplication/json, and preserve existing .png/wildcard behavior. I found no compatibility regression in the covered API. My account cannot submit an approval on this repository, so this is a factual verification comment.

[kilisamemarisaaa]

I reviewed the source change at exact head 6133c48. To verify it without changing the PR, I applied the PR's src/index.ts diff to an isolated worktree at the PR base and ran:\n\n-
pm test: 47 passing (100% statements/functions/lines, 95.91% branches)\n-
pm run build\n-
px --no-install prettier --check README.md src/index.ts src/index.spec.ts\n- git diff --check\n\nA built-artifact smoke test confirms is() and
equest() return js/.js for �pplication/javascript, �pplication/x-javascript, and ext/javascript, reject �pplication/json, and preserve existing .png/wildcard behavior. I found no compatibility regression in the covered API. My account cannot submit an approval on this repository, so this is a factual verification comment.