Add parse-bridges CLI subcommand

[integer-overflown]

What's this?

A new parse-bridges subcommand.
Also, an update to the documentation in the example showing this feature's preview.

This is handy for usage in higher-level build systems.
For example, in my case, I build both Rust and Swift with CMake, and having this CLI command implemented would allow generating the glue code from CMake as well, as a dependency step before building Rust/Swift targets (via add_custom_target API).

Notes

One caveat of the current implementation is that one would have to duplicate the crate name in the invocation line (the first argument).

This is fine for cases like mine, where this would be taken from the build system anyway, but it may not be handy for other cases.

The package name can be automatically deduced if one's running in the correct directory (the package root).
In this case, we can parse the cargo read-manifest output and get the name from there.

This would require a new dependency, though (serde_json), so I decided not to do this just yet, but if this sounds okay to you, I'll go ahead and implement this as well.