Make standard commands more ergonomic (in niche cases) (#8249)

# Objective

I ran into a case where I need to create a `CommandQueue` and push
standard `Command` actions like `Insert` or `Remove` to it manually. I
saw that `Remove` looked as follows:

```rust
struct Remove<T> {
  entity: Entity,
  phantom: PhantomData<T>
}
```

so naturally, I tried to use `Remove::<Foo>::from(entity)` but it didn't
exist. We need to specify the `PhantomData` explicitly when creating
this command action. The same goes for `RemoveResource` and
`InitResource`

## Solution

This PR implements the following:

- `From<Entity>` for `Remove<T>`
- `Default` for `RemoveResource` and `InitResource`
- use these traits in the implementation of methods of `Commands`
- rename `phantom` field on the structs above to `_phantom` to have a
more uniform field naming scheme for the command actions

---

## Changelog

> This section is optional. If this was a trivial fix, or has no
externally-visible impact, you can delete this section.

- Added: implemented `From<Entity>` for `Remove<T>` and `Default` for
`RemoveResource` and `InitResource` for ergonomics

---------

Co-authored-by: Alice Cecile <[email protected]>

crates/bevy_ecs/src/system/commands/mod.rs

@@ -444,9 +444,7 @@ impl<'w, 's> Commands<'w, 's> {
     /// # bevy_ecs::system::assert_is_system(initialise_scoreboard);
     /// ```
     pub fn init_resource<R: Resource + FromWorld>(&mut self) {
-        self.queue.push(InitResource::<R> {
-            _phantom: PhantomData::<R>::default(),
-        });
+        self.queue.push(InitResource::<R>::new());
     }
 
     /// Pushes a [`Command`] to the queue for inserting a [`Resource`] in the [`World`] with a specific value.
@@ -499,9 +497,7 @@ impl<'w, 's> Commands<'w, 's> {
     /// # bevy_ecs::system::assert_is_system(system);
     /// ```
     pub fn remove_resource<R: Resource>(&mut self) {
-        self.queue.push(RemoveResource::<R> {
-            phantom: PhantomData,
-        });
+        self.queue.push(RemoveResource::<R>::new());
     }
 
     /// Pushes a generic [`Command`] to the command queue.
@@ -748,10 +744,7 @@ impl<'w, 's, 'a> EntityCommands<'w, 's, 'a> {
     where
         T: Bundle,
     {
-        self.commands.add(Remove::<T> {
-            entity: self.entity,
-            phantom: PhantomData,
-        });
+        self.commands.add(Remove::<T>::new(self.entity));
         self
     }
 
@@ -956,6 +949,16 @@ where
     }
 }
 
+impl<T> Remove<T> {
+    /// Creates a [`Command`] which will remove the specified [`Entity`] when flushed
+    pub const fn new(entity: Entity) -> Self {
+        Self {
+            entity,
+            phantom: PhantomData::<T>,
+        }
+    }
+}
+
 pub struct InitResource<R: Resource + FromWorld> {
     _phantom: PhantomData<R>,
 }
@@ -966,6 +969,15 @@ impl<R: Resource + FromWorld> Command for InitResource<R> {
     }
 }
 
+impl<R: Resource + FromWorld> InitResource<R> {
+    /// Creates a [`Command`] which will insert a default created [`Resource`] into the [`World`]
+    pub const fn new() -> Self {
+        Self {
+            _phantom: PhantomData::<R>,
+        }
+    }
+}
+
 pub struct InsertResource<R: Resource> {
     pub resource: R,
 }
@@ -986,6 +998,15 @@ impl<R: Resource> Command for RemoveResource<R> {
     }
 }
 
+impl<R: Resource> RemoveResource<R> {
+    /// Creates a [`Command`] which will remove a [`Resource`] from the [`World`]
+    pub const fn new() -> Self {
+        Self {
+            phantom: PhantomData::<R>,
+        }
+    }
+}
+
 /// [`Command`] to log the components of a given entity. See [`EntityCommands::log_components`].
 pub struct LogComponents {
     entity: Entity,