This PR adds two new CI workflows `release` and `release-on-tag`.
The release process and artifacts are described in `docs/internal/release.md`,
so I won't repeat it here.

The other change made in this PR is the addition of `package.rust_version`
property to `Cargo.toml` of the library crates `marker_api`, `marker_uitest`,
`marker_utils`.

I was thinking to refactor the layout of the crates in the repo to separate
the library crates that have a strict MSRV, and the pre-compiled crates that
depend on the pinned nightly toolchain version (rust-marker#193 (comment)),
but I decided to do that for now. It's not a big deal, and I don't want to
extend this PR with even more changes for that.

The scope of this PR doesn't include the code changes in `cargo-marker` to
make it use the pre-compiled `marker_rustc_driver` from the GitHub release artifacts,
falling back to `cargo install` if that doesn't work. That should be implemented in
the scope of rust-marker#238.

… script.

I tested the pre-releases with `1.0.0-rc.1` version and it didn't work because
the regex didn't include the `.1` part, now it's fixed.