Make DebugName work when building with no default features (#19824)

# Objective

Let `bevy_utils` build with no default features.  

`cargo build -p bevy_utils --no-default-features` currently fails with 

```
error[E0433]: failed to resolve: use of unresolved module or unlinked crate `alloc`
 --> crates\bevy_utils\src\debug_info.rs:1:5
  |
1 | use alloc::{borrow::Cow, fmt, string::String};
  |     ^^^^^ use of unresolved module or unlinked crate `alloc`
  |
  = help: add `extern crate alloc` to use the `alloc` crate

error[E0432]: unresolved import `alloc`
 --> crates\bevy_utils\src\debug_info.rs:1:5
  |
1 | use alloc::{borrow::Cow, fmt, string::String};
  |     ^^^^^ help: a similar path exists: `core::alloc`
```

I would have expected CI to catch this earlier, but I have not
investigated why it did not.

## Solution

Wrap the parts of `DebugName` that use `Cow` and `String` in
`cfg::alloc!`.

If the `debug` feature is enabled, then `DebugName` itself stores a
`Cow`, so make the `debug` feature require `bevy_platform/alloc`.

That is, you can use `DebugName` in no-std contexts when it's just a
ZST! (I bet it's even possible to support no-std `debug` by storing
`&'static str` instead of `Cow<'static, str>`, but that seemed like too
much complexity for now.)

crates/bevy_utils/Cargo.toml

@@ -18,7 +18,7 @@ parallel = ["bevy_platform/std", "dep:thread_local"]
 
 std = ["disqualified/alloc"]
 
-debug = []
+debug = ["bevy_platform/alloc"]
 
 [dependencies]
 bevy_platform = { path = "../bevy_platform", version = "0.17.0-dev", default-features = false }

crates/bevy_utils/src/debug_info.rs

@@ -1,4 +1,7 @@
-use alloc::{borrow::Cow, fmt, string::String};
+use crate::cfg;
+cfg::alloc! {
+    use alloc::{borrow::Cow, fmt, string::String};
+}
 #[cfg(feature = "debug")]
 use core::any::type_name;
 use disqualified::ShortName;
@@ -16,7 +19,8 @@ pub struct DebugName {
     name: Cow<'static, str>,
 }
 
-impl fmt::Display for DebugName {
+cfg::alloc! {
+    impl fmt::Display for DebugName {
         fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
             #[cfg(feature = "debug")]
             f.write_str(self.name.as_ref())?;
@@ -25,6 +29,7 @@ impl fmt::Display for DebugName {
 
             Ok(())
         }
+    }
 }
 
 impl DebugName {
@@ -39,6 +44,7 @@ impl DebugName {
         }
     }
 
+    cfg::alloc! {
         /// Create a new `DebugName` from a `String`
         ///
         /// The value will be ignored if the `debug` feature is not enabled
@@ -49,6 +55,7 @@ impl DebugName {
                 name: Cow::Owned(value),
             }
         }
+    }
 
     /// Create a new `DebugName` from a type by using its [`core::any::type_name`]
     ///
@@ -79,7 +86,8 @@ impl DebugName {
     }
 }
 
-impl From<Cow<'static, str>> for DebugName {
+cfg::alloc! {
+    impl From<Cow<'static, str>> for DebugName {
         #[cfg_attr(not(feature = "debug"), expect(unused_variables))]
         fn from(value: Cow<'static, str>) -> Self {
             Self {
@@ -87,12 +95,13 @@ impl From<Cow<'static, str>> for DebugName {
                 name: value,
             }
         }
-}
+    }
 
-impl From<String> for DebugName {
+    impl From<String> for DebugName {
         fn from(value: String) -> Self {
             Self::owned(value)
         }
+    }
 }
 
 impl From<&'static str> for DebugName {