replace Travis-CI with GitHub Actions

[SethTisue]

context: scala/scala-dev#785

[SethTisue]

this is still very WIPpy at the moment

this module has the weirdest build and CI setup of any of the modules, so it's going to take some extra thought and care

• add publishing secrets to Actions settings
• replace ci build command
• adapt/replace TEST_SCALAFIX
• adapt/replace TEST_BINARY_COMPAT
• adapt/replace TEST_SCALAFMT and the scalafmt-test and scalafmt build commands
  • admin directory had install-scalafmt-githook.sh and pre-commit and scalafmt.sh but I really don't want all that, surely we can do it some simpler way?
• I downgraded to scalafix 0.9.30 for now because with 0.9.31 it hit some 2.12.14 vs 2.12.15 thing I didn't understand or dig into yet
• I further downgraded to scalafix 0.9.29 for now because scalafix-tests failed and I don't know why

[SethTisue]

well, this got out of hand.

resubmitting some of this stuff at #486, while still staying on Travis-CI

then we can switch CI systems in another followup PR

[SethTisue]

note that the problems I thought I was having with the scalafix version were actually because because setting excludeFilter to make headerCheck pass was breaking scalafixTests. in #487 I'm skipping headerCheck when it's unwanted with headerCheck := Nil instead