* ci: parallelize CI jobs
- remove the dependency on the lint job from all other jobs
- implement workflow concurrency
- reorder the workflow so that the lint, clippy and coverage jobs are
scheduled before the test jobs
- run jobs which run for each backend in parallel by calling e.g.
cargo make test-termion, instead of cargo make test
- add a coverage task to the makefile
- change "cargo-make check" to check all features valid for OS in
parallel
- run clippy only on the ubuntu-latest runner and check all features
valid in parallel
- tidy up the workflow file
* ci: simplify Makefile OS detection
Use platform overrides to significantly simplify the Makefile logic
See https://github.com/sagiegurari/cargo-make\#platform-override
* fix(termwiz): skip doc test that requires stdout
bb061fdab6