moving to re2j 2.0

[James Ring]

Hi all,

I'm planning on making some changes to re2j that will require Java 9+
to build and Java 8+ to run re2j. Previously we've supported running
on Java 7, so this is a significant change that deserves a new major
release number. I don't think many people are affected, Java 7 reached
its end-of-life more than five years ago now.

I plan to port the tests to JUnit 5 and update a bunch of tools to
more recent versions (e.g Error Prone 2.4.0).

After that, I'd like to generate UnicodeTables at runtime on demand,
so that RE2J and the JVM use the same version of Unicode. Today, RE2J
uses Unicode 6.0 even if it's running on a JVM with a different
Unicode version.

I will backport significant fixes in re2j to the 1.4 branch for the
time being, at least until doing so becomes onerous. :)

Thanks,
James