feat: add language detector

[saurabhhhcodes]

Summary

• add a dependency-free detectLanguage() helper for prompt/content inputs
• detect common code and content languages with regex and keyword signals
• scan bounded input only and return unknown for low-signal text
• export the detector and result type from the package entrypoint
• add focused detector tests

Validation

• npm ci
• npm run build
• node tests/language-detector.test.js
• node tests/cross-provider.test.js
• git diff --check

Closes #19

[saurabhhhcodes]

Quick review/scoring follow-up: this PR is clean from my side and has no checks configured in the repo.

Validation already run:

• npm ci
• npm run build
• node tests/language-detector.test.js
• node tests/cross-provider.test.js
• git diff --check

Could you please review when you get a chance and add the appropriate GSSoC labels before merge? The linked issue is #19 and the PR currently has no visible scoring labels.