gpui: Simplify Action macros + support doc comments in actions! (#33263)

Instead of a menagerie of macros for implementing `Action`, now there
are just two:

* `actions!(editor, [MoveLeft, MoveRight])`
* `#[derive(..., Action)]` with `#[action(namespace = editor)]`

In both contexts, `///` doc comments can be provided and will be used in
`JsonSchema`.

In both contexts, parameters can provided in `#[action(...)]`:

- `namespace = some_namespace` sets the namespace. In Zed this is
required.

- `name = "ActionName"` overrides the action's name. This must not
contain "::".

- `no_json` causes the `build` method to always error and
`action_json_schema` to return `None`
and allows actions not implement `serde::Serialize` and
`schemars::JsonSchema`.

- `no_register` skips registering the action. This is useful for
implementing the `Action` trait
while not supporting invocation by name or JSON deserialization.

- `deprecated_aliases = ["editor::SomeAction"]` specifies deprecated old
names for the action.
These action names should *not* correspond to any actions that are
registered. These old names
can then still be used to refer to invoke this action. In Zed, the
keymap JSON schema will
accept these old names and provide warnings.

- `deprecated = "Message about why this action is deprecation"`
specifies a deprecation message.
In Zed, the keymap JSON schema will cause this to be displayed as a
warning. This is a new feature.

Also makes the following changes since this seems like a good time to
make breaking changes:

* In `zed.rs` tests adds a test with an explicit list of namespaces. The
rationale for this is that there is otherwise no checking of `namespace
= ...` attributes.

* `Action::debug_name` renamed to `name_for_type`, since its only
difference with `name` was that it

* `Action::name` now returns `&'static str` instead of `&str` to match
the return of `name_for_type`. This makes the action trait more limited,
but the code was already assuming that `name_for_type` is the same as
`name`, and it requires `&'static`. So really this just makes the trait
harder to misuse.

* Various action reflection methods now use `&'static str` instead of
`SharedString`.

Release Notes:

- N/A

crates/gpui_macros/src/gpui_macros.rs

@@ -1,3 +1,4 @@
+mod derive_action;
 mod derive_app_context;
 mod derive_into_element;
 mod derive_render;
@@ -12,12 +13,18 @@ mod derive_inspector_reflection;
 use proc_macro::TokenStream;
 use syn::{DeriveInput, Ident};
 
-/// register_action! can be used to register an action with the GPUI runtime.
-/// You should typically use `gpui::actions!` or `gpui::impl_actions!` instead,
-/// but this can be used for fine grained customization.
+/// `Action` derive macro - see the trait documentation for details.
+#[proc_macro_derive(Action, attributes(action))]
+pub fn derive_action(input: TokenStream) -> TokenStream {
+    derive_action::derive_action(input)
+}
+
+/// This can be used to register an action with the GPUI runtime when you want to manually implement
+/// the `Action` trait. Typically you should use the `Action` derive macro or `actions!` macro
+/// instead.
 #[proc_macro]
 pub fn register_action(ident: TokenStream) -> TokenStream {
-    register_action::register_action_macro(ident)
+    register_action::register_action(ident)
 }
 
 /// #[derive(IntoElement)] is used to create a Component out of anything that implements