Add userHome, currentWorkingDirectory to Files API

[armanbilge]

Not entirely sure if Files is the best place for these.

Maybe after typelevel/cats-effect#2650 lands we can add a test that compares to env.

[ChristopherDavenport]

Really looking forward to this, with this plus some env/os-detect actual applications can be written in pure style super straight-forward. Where we can customize where we run, where we save data, etc.

[rossabaker]

java.io.tmpdir is another meaningful one to the JVM, but I don't know what the Scala.js equivalent would be. Don't block this on that if it's not obvious.

[armanbilge]

We already have facilities for creating temporary files and temporary directories. Is it helpful to expose the tmp dir directly like this?

[rossabaker]

It's useful for providing the default, but that's already tucked away behind an Option. Distraction withdrawn.