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

crates/nu-protocol/src/plugin/identity.rs

@@ -1,11 +1,10 @@
 use std::path::{Path, PathBuf};
 
-use crate::{ParseError, Spanned};
+use crate::{ParseError, ShellError, Spanned};
 
-/// Error when an invalid plugin filename was encountered. This can be converted to [`ParseError`]
-/// if a span is added.
+/// Error when an invalid plugin filename was encountered.
 #[derive(Debug, Clone)]
-pub struct InvalidPluginFilename;
+pub struct InvalidPluginFilename(PathBuf);
 
 impl std::fmt::Display for InvalidPluginFilename {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
@@ -23,6 +22,18 @@ impl From<Spanned<InvalidPluginFilename>> for ParseError {
     }
 }
 
+impl From<Spanned<InvalidPluginFilename>> for ShellError {
+    fn from(error: Spanned<InvalidPluginFilename>) -> ShellError {
+        ShellError::GenericError {
+            error: format!("Invalid plugin filename: {}", error.item.0.display()),
+            msg: "not a valid plugin filename".into(),
+            span: Some(error.span),
+            help: Some("valid Nushell plugin filenames must start with `nu_plugin_`".into()),
+            inner: vec![],
+        }
+    }
+}
+
 #[derive(Debug, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
 pub struct PluginIdentity {
     /// The filename used to start the plugin
@@ -35,17 +46,25 @@ pub struct PluginIdentity {
 
 impl PluginIdentity {
     /// Create a new plugin identity from a path to plugin executable and shell option.
+    ///
+    /// The `filename` must be an absolute path. Canonicalize before trying to construct the
+    /// [`PluginIdentity`].
     pub fn new(
         filename: impl Into<PathBuf>,
         shell: Option<PathBuf>,
     ) -> Result<PluginIdentity, InvalidPluginFilename> {
-        let filename = filename.into();
+        let filename: PathBuf = filename.into();
+
+        // Must pass absolute path.
+        if filename.is_relative() {
+            return Err(InvalidPluginFilename(filename));
+        }
 
         let name = filename
             .file_stem()
             .map(|stem| stem.to_string_lossy().into_owned())
             .and_then(|stem| stem.strip_prefix("nu_plugin_").map(|s| s.to_owned()))
-            .ok_or(InvalidPluginFilename)?;
+            .ok_or_else(|| InvalidPluginFilename(filename.clone()))?;
 
         Ok(PluginIdentity {
             filename,
@@ -89,30 +108,42 @@ impl PluginIdentity {
             .expect("fake plugin identity path is invalid")
     }
 
-    /// A command that could be used to register the plugin, for suggesting in errors.
-    pub fn register_command(&self) -> String {
+    /// A command that could be used to add the plugin, for suggesting in errors.
+    pub fn add_command(&self) -> String {
         if let Some(shell) = self.shell() {
             format!(
-                "register --shell '{}' '{}'",
+                "plugin add --shell '{}' '{}'",
                 shell.display(),
                 self.filename().display(),
             )
         } else {
-            format!("register '{}'", self.filename().display())
+            format!("plugin add '{}'", self.filename().display())
         }
     }
+
+    /// A command that could be used to reload the plugin, for suggesting in errors.
+    pub fn use_command(&self) -> String {
+        format!("plugin use '{}'", self.name())
+    }
 }
 
 #[test]
 fn parses_name_from_path() {
     assert_eq!("test", PluginIdentity::new_fake("test").name());
     assert_eq!("test_2", PluginIdentity::new_fake("test_2").name());
+    let absolute_path = if cfg!(windows) {
+        r"C:\path\to\nu_plugin_foo.sh"
+    } else {
+        "/path/to/nu_plugin_foo.sh"
+    };
     assert_eq!(
         "foo",
-        PluginIdentity::new("nu_plugin_foo.sh", Some("sh".into()))
+        PluginIdentity::new(absolute_path, Some("sh".into()))
             .expect("should be valid")
             .name()
     );
+    // Relative paths should be invalid
+    PluginIdentity::new("nu_plugin_foo.sh", Some("sh".into())).expect_err("should be invalid");
     PluginIdentity::new("other", None).expect_err("should be invalid");
     PluginIdentity::new("", None).expect_err("should be invalid");
 }