Deprecate register and add plugin use (#12607)

# Description

Adds a new keyword, `plugin use`. Unlike `register`, this merely loads
the signatures from the plugin cache file. The file is configurable with
the `--plugin-config` option either to `nu` or to `plugin use` itself,
just like the other `plugin` family of commands. At the REPL, one might
do this to replace `register`:

```nushell
> plugin add ~/.cargo/bin/nu_plugin_foo
> plugin use foo
```

This will not work in a script, because `plugin use` is a keyword and
`plugin add` does not evaluate at parse time (intentionally). This means
we no longer run random binaries during parse.

The `--plugins` option has been added to allow running `nu` with certain
plugins in one step. This is used especially for the `nu_with_plugins!`
test macro, but I'd imagine is generally useful. The only weird quirk is
that it has to be a list, and we don't really do this for any of our
other CLI args at the moment.

`register` now prints a deprecation parse warning.

This should fix #11923, as we now have a complete alternative to
`register`.

# User-Facing Changes

- Add `plugin use` command
- Deprecate `register`
- Add `--plugins` option to `nu` to replace a common use of `register`

# Tests + Formatting

I think I've tested it thoroughly enough and every existing test passes.
Testing nu CLI options and alternate config files is a little hairy and
I wish there were some more generic helpers for this, so this will go on
my TODO list for refactoring.

- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`

# After Submitting

- [ ] Update plugins sections of book
- [ ] Release notes

tests/plugins/nu_plugin_nu_example.rs

@@ -1,26 +1,42 @@
-use nu_test_support::nu;
-
-#[test]
-fn register() {
-    let out = nu!("register crates/nu_plugin_nu_example/nu_plugin_nu_example.nu");
-    assert!(out.status.success());
-    assert!(out.out.trim().is_empty());
-    assert!(out.err.trim().is_empty());
-}
+use assert_cmd::Command;
 
 #[test]
 fn call() {
-    let out = nu!(r#"
-        register crates/nu_plugin_nu_example/nu_plugin_nu_example.nu
-        nu_plugin_nu_example 4242 teststring
-    "#);
-    assert!(out.status.success());
+    // Add the `nu` binaries to the path env
+    let path_env = std::env::join_paths(
+        std::iter::once(nu_test_support::fs::binaries()).chain(
+            std::env::var_os(nu_test_support::NATIVE_PATH_ENV_VAR)
+                .as_deref()
+                .map(std::env::split_paths)
+                .into_iter()
+                .flatten(),
+        ),
+    )
+    .expect("failed to make path var");
 
-    assert!(out.err.contains("name: nu_plugin_nu_example"));
-    assert!(out.err.contains("4242"));
-    assert!(out.err.contains("teststring"));
+    let assert = Command::new(nu_test_support::fs::executable_path())
+        .env(nu_test_support::NATIVE_PATH_ENV_VAR, path_env)
+        .args([
+            "--no-config-file",
+            "--no-std-lib",
+            "--plugins",
+            &format!(
+                "[crates{0}nu_plugin_nu_example{0}nu_plugin_nu_example.nu]",
+                std::path::MAIN_SEPARATOR
+            ),
+            "--commands",
+            "nu_plugin_nu_example 4242 teststring",
+        ])
+        .assert()
+        .success();
 
-    assert!(out.out.contains("one"));
-    assert!(out.out.contains("two"));
-    assert!(out.out.contains("three"));
+    let output = assert.get_output();
+    let stdout = String::from_utf8_lossy(&output.stdout);
+    let stderr = String::from_utf8_lossy(&output.stderr);
+    assert!(stdout.contains("one"));
+    assert!(stdout.contains("two"));
+    assert!(stdout.contains("three"));
+    assert!(stderr.contains("name: nu_plugin_nu_example"));
+    assert!(stderr.contains("4242"));
+    assert!(stderr.contains("teststring"));
 }