Split the plugin crate (#12563)

# Description

This breaks `nu-plugin` up into four crates:

- `nu-plugin-protocol`: just the type definitions for the protocol, no
I/O. If someone wanted to wire up something more bare metal, maybe for
async I/O, they could use this.
- `nu-plugin-core`: the shared stuff between engine/plugin. Less stable
interface.
- `nu-plugin-engine`: everything required for the engine to talk to
plugins. Less stable interface.
- `nu-plugin`: everything required for the plugin to talk to the engine,
what plugin developers use. Should be the most stable interface.

No changes are made to the interface exposed by `nu-plugin` - it should
all still be there. Re-exports from `nu-plugin-protocol` or
`nu-plugin-core` are used as required. Plugins shouldn't ever have to
use those crates directly.

This should be somewhat faster to compile as `nu-plugin-engine` and
`nu-plugin` can compile in parallel, and the engine doesn't need
`nu-plugin` and plugins don't need `nu-plugin-engine` (except for test
support), so that should reduce what needs to be compiled too.

The only significant change here other than splitting stuff up was to
break the `source` out of `PluginCustomValue` and create a new
`PluginCustomValueWithSource` type that contains that instead. One bonus
of that is we get rid of the option and it's now more type-safe, but it
also means that the logic for that stuff (actually running the plugin
for custom value ops) can live entirely within the `nu-plugin-engine`
crate.

# User-Facing Changes
- New crates.
- Added `local-socket` feature for `nu` to try to make it possible to
compile without that support if needed.

# Tests + Formatting
- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`

src/main.rs

@@ -391,7 +391,7 @@ fn main() -> Result<()> {
 
     #[cfg(feature = "plugin")]
     if let Some(plugins) = &parsed_nu_cli_args.plugins {
-        use nu_plugin::{GetPlugin, PluginDeclaration};
+        use nu_plugin_engine::{GetPlugin, PluginDeclaration};
         use nu_protocol::{engine::StateWorkingSet, ErrSpan, PluginIdentity};
 
         // Load any plugins specified with --plugins
@@ -409,7 +409,7 @@ fn main() -> Result<()> {
                 .map_err(ShellError::from)?;
 
             // Create the plugin and add it to the working set
-            let plugin = nu_plugin::add_plugin_to_working_set(&mut working_set, &identity)?;
+            let plugin = nu_plugin_engine::add_plugin_to_working_set(&mut working_set, &identity)?;
 
             // Spawn the plugin to get its signatures, and then add the commands to the working set
             for signature in plugin.clone().get_plugin(None)?.get_signature()? {