feat: Add cargo component add command

[saulecabrera]

Implements #5

This commit introduces the add command, analogous to cargo add for
adding dependendcies to the manifest.

This command validates:

1. Name conflicts between dependencies
2. Only 1 default export can exist

---

For reference, this the help output of the subcommand:

cargo-component-add 0.1.0
Add a depency for a WebAssembly component

USAGE:
    cargo component add [OPTIONS] --path <PATH> <name>

ARGS:
    <name>    Name of the dependency

OPTIONS:
        --color <WHEN>            Coloring: auto, always, never
        --dry-run                 Don't actually write the manifest
        --export                  Set the dependency as an exported interface
    -h, --help                    Print help information
        --manifest-path <PATH>    Path to Cargo.toml
    -p, --package <SPEC>          Package to add the dependency to (see `cargo help pkgid`)
        --path <PATH>             Path to the interface definition of the dependency
    -q, --quiet                   Do not print cargo log messages
    -v, --verbose                 Use verbose output (-vv very verbose/build.rs output)
        --version <VERSION>       Set the version of the dependency

---

Some open questions around the design:

1. The command is currently not validating the existence/validity of the .wit file; I left it this way operating under the assumption that it's a concern of the build command to validate the existence of the interface file. Also, I can see a possibility of this command being used as a starting point to get the Cargo.toml changes ready, rather than as a final step. That said, if you think this should be validated at this step, I can definitely add that in.
2. The --dry-run option currently prints the un-written state of the manifest to the stdout, not sure if there's a better course of action here.

[peterhuene]

This looks great to me, just a few comments.

Regarding the validation of the interface file, I think we might as well check it at least exists when adding it to Cargo.toml; as to whether or not it parses as an interface definition, I'm fine with leaving that up to the other commands that need to read the file contents.

[saulecabrera]

@peterhuene thanks for the review, this should be ready for another pass. I also realized that in my original implementation I was not validating the presence of the version if the dependency is an import. I added that in 35481c2

[peterhuene]

👍