Replace println! calls with snapshot tests

[spenserblack]

As discussed in #156, this converts the visual tests into things that can be asserted with cargo test. These can still be visually reviewed with cat path/to/snapshot. This also registers the detected language for *.snap files as plain text -- currently, GitHub will detect them as Jest snapshot files and apply a JavaScript-like syntax highlighting otherwise. This also installs cargo-insta when a contributor creates a new codespace.

The one println! in a test that I didn't address is here

colored/src/lib.rs

Line 839 in 682adf8

println!("first: {}\nsecond: {}", expected, output);

Since that didn't look like part of the test, but just a snippet to help debug. I was tempted to switch it to a dbg! call, but dbg! prints the Debug formatting instead of the display formatting.

[kurtlawrence]

LGTM! Thanks for the hard work!

Cna merge in once the CI passes

[spenserblack]

Oh, interesting. The CI environment didn't detect truecolor support by default, causing the snapshot tests to fail. I updated the env key in the test workflow to force true color. Perhaps there should be some sort of togglable value, even if it's internal, so that this can be forced inside tests instead of leaving it up to the environment.