Add script to find missing tokens

[IAmThePan]

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I created a tool to search the extension for translation tokens and check to make sure those tokens are in the default messages.json file.
Run the tool by running node tools/token-checker.js.
Requires one new package under dev dependencies: find-in-files.

This tool does NOT work for tokens that are formed using variable names:

• ✔ t('global_settings_saved_category')
• ✘ t(alert_${type}_off)