migrate fpb-lint from travisci to gh actions

[borgified]

What does this PR do?

migrate the fpb-lint automation running on travisci to github actions

[eshellman]

can we hold off on removing travis-ci until we have enough experience to see that

• the new config does what we want
• we can modify our documentation

[borgified]

was tested here: borgified#8
i did intentionally mess up the alphabetical ordering of a file and the linter found the error as expected but you dont see that output cuz i forced pushed to remove extraneous commits.

[eshellman]

travis-ci and gh-actions-lint run on both push and pull-request; shouldn't it be the same for gh actions?
I'd really like to be able to compare the two side-by-side - will that cause any problems?

[borgified]

i think they both run on pull_request based on what ive seen on a few randomly selected PRs. (eg. https://github.com/EbookFoundation/free-programming-books/pull/5188/checks)

as configured, it will re-check if a change is made to the pull request.

[eshellman]

i'm just comparing the config files for the linter action and the link-check action and not that the "on" parameter is different. I've not so much as read the actions documentation, though.

[eshellman]

merged, but restored the travis config so we can compare results and not confuse folks before documentation has changed