Permit explicit target opt-in

[CorvusPrudens]

This PR supersedes #212 with a more general solution. The utility is explained there.

Rather than providing a single --bin-skip flag, the build command now has two additional flags --bin and --lib. These provide explicit opt-ins for their respective targets.

The default behavior is to build both. If an explicit opt-in is provided for one target, the other target will be excluded unless it also receives an opt-in.

This behavior might be a bit confusing. If there's a better and simpler approach, for example simply providing a boolean value for each, that should be an easy change.

If additional testing is required, please let me know! I simply adjusted the tests assuming no flags were provided.

[benwis]

@CorvusPrudens Sorry about the delayed review, if you can fix the conflicts I'd be happy to merge this!

[CorvusPrudens]

Looking at the CLI flags again, I think it may make more sense to take a boolean value.

For example:

cargo leptos build --bin false

There are already precedents for boolean options right in the build subcommand, and it makes the functionality more clear.

I should be able to implement this very soon. It's really not much different from how it works already.