gpui-macros: Hide autogenerated action types/functions (#10417)

I've found it a bit annoying that autogenerated code shows up in
completions when working on Zed, so I've moved them into an associated
function of a struct we're "implementing" Action on. That way, neither a
generated function nor a static show up in completions.

Note that this change only affects Zed codebase! I'm pushing it up right
after last Preview to give it some time on Nightly.
Before:

![image](https://github.com/zed-industries/zed/assets/24362066/7343201f-f05b-4342-a9f7-97f002d88a48)

![image](https://github.com/zed-industries/zed/assets/24362066/e67f9dcb-e090-40e0-873c-e51bd39e0c7e)

After:

![image](https://github.com/zed-industries/zed/assets/24362066/0704211a-73f5-4f12-8583-9e47f092e5b7)

![image](https://github.com/zed-industries/zed/assets/24362066/c6fa00f5-fd8f-4f06-8be7-b74acedccd7c)


Release Notes:

- N/A

crates/gpui/src/action.rs

@@ -182,7 +182,9 @@ impl ActionRegistry {
 macro_rules! actions {
     ($namespace:path, [ $($name:ident),* $(,)? ]) => {
         $(
-            /// The `$name` action see [`gpui::actions!`]
+            #[doc = "The `"]
+            #[doc = stringify!($name)]
+            #[doc = "` action, see [`gpui::actions!`]"]
             #[derive(::std::cmp::PartialEq, ::std::clone::Clone, ::std::default::Default, ::std::fmt::Debug, gpui::private::serde_derive::Deserialize)]
             #[serde(crate = "gpui::private::serde")]
             pub struct $name;

crates/gpui_macros/src/register_action.rs

@@ -22,11 +22,11 @@ pub(crate) fn register_action(type_name: &Ident) -> proc_macro2::TokenStream {
     );
 
     quote! {
+        impl #type_name {
+            /// This is an auto generated function, do not use.
+            #[automatically_derived]
             #[doc(hidden)]
-        #[gpui::private::linkme::distributed_slice(gpui::__GPUI_ACTIONS)]
-        #[linkme(crate = gpui::private::linkme)]
-        static #static_slice_name: gpui::MacroActionBuilder = #action_builder_fn_name;
-
+            fn __autogenerated() {
                 /// This is an auto generated function, do not use.
                 #[doc(hidden)]
                 fn #action_builder_fn_name() -> gpui::ActionData {
@@ -36,5 +36,13 @@ pub(crate) fn register_action(type_name: &Ident) -> proc_macro2::TokenStream {
                         build: <#type_name as gpui::Action>::build,
                     }
                 }
+                #[doc(hidden)]
+                #[gpui::private::linkme::distributed_slice(gpui::__GPUI_ACTIONS)]
+                #[linkme(crate = gpui::private::linkme)]
+                static #static_slice_name: gpui::MacroActionBuilder = #action_builder_fn_name;
+            }
+        }
+
+
     }
 }