Implement --matrix-extra for more matrix dimensions

Sometimes you need to test your project along more dimensions than
just GHC version.  This is particularly important for
programs/libraries that use FFI to bind to libraries - they might
need to be tested against a range of library versions.

In general, you want to test all the combinations of GHC versions
and other properties, i.e. the cartesian product.  It is burdensome
for maintainers that need such a strategy to manually adjust the
matrix after every (re)generation of the CI script/spec.  Better
tool support for this scenario is warranted.

This commit implements a new --matrix-extra option, which adds
additional matrix dimensions.  The option value format is:

  --matrix-extra libfoo:2.6,3.0;libbar:0.1,0.2

haskell-ci adds all the combinations of GHC version and the
--matrix-extra fields to the matrix.  Corresponding build/test steps
can be introduced via --github-patches (or --travis-patches).

This commit implements this feature for GitHub actions only.
Additionally, we check and enforce the job limit imposed by GitHub
(currently 256 jobs per workflow).

Author: frasertweedale

fixtures/matrix-extra-toomany.args

@@ -0,0 +1,2 @@
+--matrix-extra=abc:1,2,3
+--matrix-extra=foo:bar,baz