Improve bevy_input_focus (#16749)

# Objective

I was curious to use the newly created `bevy_input_focus`, but I found
some issues with it
  - It was only implementing traits for `World`.
  - Lack of tests
  - `is_focus_within` logic was incorrect.


## Solution
 This PR includes some improvements to the `bevy_input_focus` crate: 
- Add new `IsFocusedHelper` that doesn't require access to `&World`. It
implements `IsFocused`
- Remove `IsFocused` impl for `DeferredWorld`. Since it already
implements `Deref<Target=World>` it was just duplication of code.
- impl `SetInputFocus` for `Commands`. There was no way to use
`SetFocusCommand` directly. This allows it.
- The `is_focus_within` logic has been fixed to check descendants.
Previously it was checking if any of the ancestors had focus which is
not correct according to the documentation.
  - Added a bunch of unit tests to verify the logic of the crate.

## Testing

- Did you test these changes? If so, how? Yes, running newly added unit
tests.

---

Author: eckz

crates/bevy_input_focus/Cargo.toml

@@ -6,17 +6,19 @@ description = "Keyboard focus management"
 homepage = "https://bevyengine.org"
 repository = "https://github.com/bevyengine/bevy"
 license = "MIT OR Apache-2.0"
-keywords = ["bevy", "color"]
+keywords = ["bevy"]
 rust-version = "1.76.0"
 
 [dependencies]
 bevy_app = { path = "../bevy_app", version = "0.15.0-dev", default-features = false }
 bevy_ecs = { path = "../bevy_ecs", version = "0.15.0-dev", default-features = false }
 bevy_input = { path = "../bevy_input", version = "0.15.0-dev", default-features = false }
 bevy_hierarchy = { path = "../bevy_hierarchy", version = "0.15.0-dev", default-features = false }
-bevy_utils = { path = "../bevy_utils", version = "0.15.0-dev", default-features = false }
 bevy_window = { path = "../bevy_window", version = "0.15.0-dev", default-features = false }
 
+[dev-dependencies]
+smol_str = "0.2"
+
 [lints]
 workspace = true