GitHub Action to phplint the code

[cclauss]

https://github.com/overtrue/phplint

Test results: https://github.com/cclauss/h5p-wordpress-plugin/actions

Because Travis CI seems to be on vacation https://travis-ci.org/github/h5p

Also, Fix typos in readme to test new phplint release

[bernhardkaindl]

@icc, can you review this or somebody else?