fix(isLocale): add support for validation of more valid language tags

[kwahome]

feat(isLocale): Add support for validation of more valid language tags

Implement RFC 5646 compliant regex to validate language tags and locales.

Fix for #2100

• https://www.w3.org/TR/ltli/
• https://www.w3.org/International/articles/language-tags/
• https://www.rfc-editor.org/rfc/rfc4647.html
• https://www.rfc-editor.org/rfc/rfc5646.html

Checklist

• PR contains only changes related; no stray files, etc.
• README updated (where applicable)
• Tests written (where applicable)
• References provided in PR (where applicable)

[codecov]

Codecov Report

Base: 100.00% // Head: 100.00% // No change to project coverage 👍

Coverage data is based on head (8147ed4) compared to base (43803c0).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #2189   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          106       106           
  Lines         2348      2359   +11     
  Branches       593       592    -1     
=========================================
+ Hits          2348      2359   +11     

Impacted Files	Coverage Δ
src/lib/isLocale.js	100.00% <100.00%> (ø)

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[profnandaa]

LGTM, thanks for your contrib @kwahome ! 🎉