Feature: Add support for rndebugger: uri scheme on Linux

[crloz]

Support rndebugger: URI scheme on Linux (via xdg-open):

• on Linux, use app.requestSingleInstanceLock()
• when opened the first time, the uri is read from precess.argv and the port in the URI takes precedence

Usage with Expo:

$ REACT_DEBUGGER="unset ELECTRON_RUN_AS_NODE && xdg-open 'rndebugger://set-debugger-loc?port=19000' ||" yarn start

Packagers for distributions not officially supported should add MimeType=x-scheme-handler/rndebugger; to the .desktop file

[jhen0409]

Thanks!