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`

crates/nu-cli/Cargo.toml

@@ -21,7 +21,7 @@ nu-cmd-base = { path = "../nu-cmd-base", version = "0.92.3" }
 nu-engine = { path = "../nu-engine", version = "0.92.3" }
 nu-path = { path = "../nu-path", version = "0.92.3" }
 nu-parser = { path = "../nu-parser", version = "0.92.3" }
-nu-plugin = { path = "../nu-plugin", version = "0.92.3", optional = true }
+nu-plugin-engine = { path = "../nu-plugin-engine", version = "0.92.3", optional = true }
 nu-protocol = { path = "../nu-protocol", version = "0.92.3" }
 nu-utils = { path = "../nu-utils", version = "0.92.3" }
 nu-color-config = { path = "../nu-color-config", version = "0.92.3" }
@@ -45,5 +45,5 @@ uuid = { workspace = true, features = ["v4"] }
 which = { workspace = true }
 
 [features]
-plugin = ["nu-plugin"]
+plugin = ["nu-plugin-engine"]
 system-clipboard = ["reedline/system_clipboard"]

crates/nu-cli/src/config_files.rs

@@ -150,7 +150,7 @@ pub fn read_plugin_file(
 
         let mut working_set = StateWorkingSet::new(engine_state);
 
-        nu_plugin::load_plugin_file(&mut working_set, &contents, span);
+        nu_plugin_engine::load_plugin_file(&mut working_set, &contents, span);
 
         if let Err(err) = engine_state.merge_delta(working_set.render()) {
             report_error_new(engine_state, &err);