This PR serves as the preparation work for some follow-up PRs that implement a parameter-level dynamic completer.

As discussed in #16859, something like:

#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
pub enum Completion {
    Command(DeclId),
    List(NuCow<&'static [&'static str], Vec<String>>),
    Builtin(Box<dyn ArgumentCompleter>), // <------- new
}

will make the life a lot easier.

I believe it's actually feasible with the trick of #[typetag::serde(tag = "type")] and dyn-clone.
So there’s going to be quite some refactoring. To make it easier to review, I think I'll try to break it into small PRs.

In order to make parameter-level completers only return results that match the user input, IMO NuMatcher is best suited in nu-protocol/src/completion/matcher.rs, hence this PR. @ysthakur

Release notes summary - What our users need to know

Nothing

Tasks after submitting

1. Make DotNuCompletion, ExportableCompletion, CommandCompletion ... implement the trait ArgumentCompleter and move them to nu-protocol/src/completion
2. Bind them to the parameters of command use/export use/which/attr complete/... right in the signature definition
3. Clean-up get_dynamic_completion related stuff introduced in Plugin: support custom completions in command flags #16859 and fix(completion): path/directory/glob typed flag values get empty completion results #17006
4. (Optional) make those builtin completers available to user-defined nushell commands. Through new syntax or specific prefixed completer names like foo: string@"builtin exportable"