Migrate to firebase v9 modular sdk

[thatfiredev]

Description

This PR should update geofire to use the new modular firebase sdk (v9).
I simply copied the changes from #217 and made small improvements with as little code changes as possible to fix the build. These include:

• replace the uglify plugin with terser
• use the stable version of firebase v9 instead of the beta
• bump the node version on GitHub actions to 10.x (minimum required by v9)

TODO:

• Docs
• Migration guide
• Examples
• Remove exp version
• Major version bump

Code sample

[coveralls]

Coverage increased (+0.2%) to 92.377% when pulling d7392a3 on thatfiredev:ss-modular-sdk into d5250d5 on firebase:master.

[thatfiredev]

@puf this is ready for review.

Please use "Squash and Merge" when you merge it.

[puf]

I'm still a bit sad about losing so much compatibility, but I guess having a /compat path might be a bridge too far here. Folks on older/compat versions of the JS SDK should stick to the older geofire-js release, since that hasn't changed significantly in years anyway. Should we mention that in the README?

[thatfiredev]

Added it to the migration guide here: thatfiredev@654fb00

[puf]

One small nit from me, and a potential improvement to the docs. With those, this LGTM