[WIP] feat(extension): i18n support

[kiyui]

This should address the concerns in #34.

Changes:

• Did some code refactor such that all strings are centralized
• Added a .pot file with all the strings
• Add some languages for testing and initial release
• Add build script so .mo files do not need to be included (add to package.json or introduce meson)

Contributing:
Where $lang is a language such as en_US, do the following in the extension directory:

• mkdir -p locale/$lang/LC_MESSAGES
• msginit -i night-light-slider.pot -o locale/$lang/LC_MESSAGES/night-light-slider.po
• Modify locale/$lang/LC_MESSAGES/night-light-slider.po with your locale
• msgfmt locale/$lang/LC_MESSAGES/night-light-slider.po -o locale/$lang/LC_MESSAGES/night-light-slider.mo
• Submit a PR without the compiled .mo files