Add assert_is_exclusive_system function (#5275)

Add compile time check for if a system is an exclusive system. Resolves #4788 

Co-authored-by: Daniel Liu <mr.picklepinosaur@gmail.com>
Co-authored-by: Daniel Liu <danieliu3120@gmail.com>

crates/bevy_ecs/src/system/mod.rs

@@ -96,6 +96,34 @@ pub fn assert_is_system<In, Out, Params, S: IntoSystem<In, Out, Params>>(sys: S)
     }
 }
 
+/// Ensure that a given function is an exclusive system
+///
+/// This should be used when writing doc examples,
+/// to confirm that systems used in an example are
+/// valid exclusive systems
+///
+/// Passing assert
+/// ```
+/// # use bevy_ecs::prelude::World;
+/// # use bevy_ecs::system::assert_is_exclusive_system;
+/// fn an_exclusive_system(_world: &mut World) {}
+///
+/// assert_is_exclusive_system(an_exclusive_system);
+/// ```
+///
+/// Failing assert
+/// ```compile_fail
+/// # use bevy_ecs::prelude::World;
+/// # use bevy_ecs::system::assert_is_exclusive_system;
+/// fn not_an_exclusive_system(_world: &mut World, number: f32) {}
+///
+/// assert_is_exclusive_system(not_an_exclusive_system);
+/// ```
+pub fn assert_is_exclusive_system<Params, SystemType>(
+    _sys: impl IntoExclusiveSystem<Params, SystemType>,
+) {
+}
+
 #[cfg(test)]
 mod tests {
     use std::any::TypeId;