Run unit tests with Github actions CI for every git push and PR

[linrock]

Automatically runs unit tests for every git push and new PR so there's no need to manually run tests to verify if tests pass. Github actions is free to use for all public repos. It can run tests on different OSes and node.js versions.

See https://github.com/linrock/chess.js/actions for example test run outputs.

See https://github.com/jhlywa/chess.js/actions for more info.