ci: better CI hygiene

[tomtau]

closes #655

[tomtau]

@CAD97 I tried to include checking for the items discussed on the issue:

• cargo msrv could not run on a workspace, so I added the loop to check individual crates
• for cargo hack, I assume checking the feature-powerset is the most relevant/useful here
• cargo-semver-checks GH action had issues with a bootstrap, so I included a custom script; it's more for assisting manual checking right now, because currently, it reports two violations on pest_meta that Rule and Parser are no longer publicly accessible from "grammar.rs"... but that seems to be a false positive: they weren't publicly accessible in 2.1.x either.

[tomtau]

I'm planning to do a quick point release (see #702 (comment) ), so will merge this now. @pest-parser/triage @CAD97 feel free to comment for follow ups for the CI changes though!