0.1

Add CI

Nothing ran on push before, so a break only showed up when someone happened to
run composer check locally.

Tests run on 8.2, 8.3, 8.4 and 8.5, the range the package now declares. Static
analysis, coding standards and coverage each run once on 8.4 rather than across
the matrix, since none of them says anything version-specific.

The coverage job is the one that needs a driver, so xdebug is only installed
there; the rest run without one, which is markedly faster.