feat(developer): web debugger ngrok integration

Initial integration of ngrok into Keyman Developer web debugger
experience, so that the user can access their web debug session from
another device without significant configuration. By default, ngrok is
switched off, but when it is enabled and configured (user must register
an email address online I think), then Keyman Developer will start an
ngrok tunnel in the background and make it the default link in the list
of available addresses.

This also adds an option to hide the list of local addresses, which is
probably clearer for users when ngrok is active. However, as local
addresses will be faster if configured, it may be less sensible to do
this.

If the user has ngrok installed and in use already, then it is
recommended that they simply open a tunnel via the ngrok interface for
the debugger web host, rather than relying on Keyman Developer's
internal integration, as Keyman Deveoper does not do handle multiple
ngrok sessions.

The user may choose to make the ngrok tunnel window visible, which will
be helpful in error scenarios.

If Keyman Developer crashes, the ngrok tunnel may be left running, and
will need to be manually killed.