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/errors/parse_warning.rs

@@ -5,19 +5,21 @@ use thiserror::Error;
 
 #[derive(Clone, Debug, Error, Diagnostic, Serialize, Deserialize)]
 pub enum ParseWarning {
-    #[error("Deprecated: {0}")]
-    DeprecatedWarning(
-        String,
-        String,
-        #[label = "`{0}` is deprecated and will be removed in 0.90. Please use `{1}` instead, more info: https://www.nushell.sh/book/custom_commands.html"]
-        Span,
-    ),
+    #[error("Deprecated: {old_command}")]
+    #[diagnostic(help("for more info: {url}"))]
+    DeprecatedWarning {
+        old_command: String,
+        new_suggestion: String,
+        #[label("`{old_command}` is deprecated and will be removed in 0.94. Please {new_suggestion} instead")]
+        span: Span,
+        url: String,
+    },
 }
 
 impl ParseWarning {
     pub fn span(&self) -> Span {
         match self {
-            ParseWarning::DeprecatedWarning(_, _, s) => *s,
+            ParseWarning::DeprecatedWarning { span, .. } => *span,
         }
     }
 }