Qualify embedded_asset expansion with $crate:: (#11961)

# Objective

Right now, if you call `embedded_asset` with 2 arguments as a qualified
path it doesn't work (`bevy::asset::embedded_asset!(app, "foo.wgsl")` ->
"cannot find macro `embedded_asset` in this scope")

## Solution

Use `$crate::` in expansion for 2-arg case.

crates/bevy_asset/src/io/embedded/mod.rs

@@ -219,7 +219,7 @@ pub fn _embedded_asset_path(
 #[macro_export]
 macro_rules! embedded_asset {
     ($app: ident, $path: expr) => {{
-        embedded_asset!($app, "src", $path)
+        $crate::embedded_asset!($app, "src", $path)
     }};
 
     ($app: ident, $source_path: expr, $path: expr) => {{