fix(flags): add a new experimental-features flag to gate new features starting with the existing "tui" flag

[kallevmercury]

Add a new experimental-features flag:

cargo run -- --experimental-features tui --command 'ghci'

It's hidden from the help text as well but will report if the flags don't match:

cargo run -- --experimental-features bogus

Based on:
#380 (review)

• Labeled the PR with patch, minor, or major to request a version bump when it's merged.
• Updated the user manual in docs/.
• Added integration / regression tests in tests/.

[9999years]

Looks pretty good! I'd like a couple small changes, and I think we should add two tests:

1. If we start ghciwatch with ghciwatch --experimental-features asdlfkjaslkdgjs then it errors immediately instead of starting.

   Something like assert!(GhciWatchBuilder::new("tests/data/simple").with_args(["--experimental-features", "asdflkj"]).start().await.is_err());, but you can look at the other tests for examples.

2. If we start ghciwatch with ghciwatch --experimental-features tui we get a warning that it's experimental.

   Similar, with something like session.wait_for_log("literal text of the log message here").await.unwrap(). There's a fancier log matching DSL available as well; I don't think you'll need it, but you can do something like "literal log message".into_matcher().but_not(BaseMatcher::ghci_started()).

I think that'll be pretty easy, but let me know if you have trouble.

[9999years]

Oh man, that's frustrating!! Maybe we can replace this with a test for your feature when we get that merged lol

[kallevmercury]

Lol yes! That was the main issue I ran into trying to get the tests running.

[9999years]

LGTM, thanks for this!!