unify chrome and firefox extension source code

[jck]

• use parcel to build chrome and firefox extensions without duplicating the source
• use https://github.com/mozilla/webextension-polyfill instead of browser specific BrowserTabs classes
• use typescript

trying it out:

• cd brotab/extension
• yarn install
• yarn build:chrome (outputs to dist/chrome)
• yarn build:firefox (outputs to dist/firefox)