package.json for 2.3.1

ready for 2.3.0

docs: update TOC

unity

Merge remote-tracking branch 'origin/master'

Merge pull request Cysharp#11 from xin9le/join-overloads

Add ZString.Join<T>(char, XxxCollection) overloads

fix resize buffer issue(append large string).

CI

Merge pull request Cysharp#4 from Cysharp/improve-perf

1.0.0 rewrite