ci: parallelize CI jobs

[joshka]

• remove the dependency on the lint job from all other jobs
• implement workflow concurrency
• 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 valid features in parallel
• add clippy-all tasks to enable all features
• tidy up the workflow file

Fixes:

• chore: fix CI so that linting doesn't block testing #291
• Format checking in incorrect workflows #194

Second commit simplifies Makefile.toml to use platform overrides for windows to avoid termion, and makes the backend choice a parameter for tasks that actually need it (e.g. cargo make test-backend termion is now possible). Where useful, the makefile now builds features together, rather than separately. All tasks now check all features rather than some skipping the calendar, etc.

Third commit fixes an existing error in the termwiz doc tests that this hightlights (was not previously tested as the previous doc tests didn't check all backends)

Performance

• CI build time reduces to 5-6 mins from 8-10 mins
• lint and clippy results often available fairly soon (<1-2 mins) after submission instead

Review notes

• It's probably difficult to review the diff, and would likely be easier to just review the Makefile and ci.yml file
• Take a look at the Checks tab to see how the CI differs from previous CI results.

[codecov]

Codecov Report

Merging #318 (21fc736) into main (446efae) will not change coverage.
The diff coverage is n/a.

❗ Current head 21fc736 differs from pull request most recent head c4afec9. Consider uploading reports for the commit c4afec9 to get more accurate results

@@           Coverage Diff           @@
##             main     #318   +/-   ##
=======================================
  Coverage   85.00%   85.00%           
=======================================
  Files          38       38           
  Lines        8507     8507           
=======================================
  Hits         7231     7231           
  Misses       1276     1276           

Impacted Files	Coverage Δ
src/backend/termwiz.rs	0.00% <ø> (ø)

... and 1 file with indirect coverage changes