Make AssetAction::Ignore not copy assets to imported_assets (#12605)

# Objective

Lets say I have the following `.meta` file:
```RON
(
    meta_format_version: "1.0",
    asset: Ignore,
)
```
When a file is inside the `assets` directory and processing is enabled,
the processor will copy the file into `imported_assets` although it
should be ignored and therefore not copied.

## Solution

- I added a simple check that does not copy the assets if the
AssetAction is `Ignore`.


## Migration Guide

- The public `ProcessResult` enum now has a `ProcessResult::Ignore`
variant that must be handled.

crates/bevy_asset/src/processor/mod.rs

@@ -722,9 +722,7 @@ impl AssetProcessor {
                         (meta, Some(processor))
                     }
                     AssetActionMinimal::Ignore => {
-                        let meta: Box<dyn AssetMetaDyn> =
-                            Box::new(AssetMeta::<(), ()>::deserialize(&meta_bytes)?);
-                        (meta, None)
+                        return Ok(ProcessResult::Ignored);
                     }
                 };
                 (meta, meta_bytes, processor)
@@ -1038,6 +1036,7 @@ impl AssetProcessorData {
 pub enum ProcessResult {
     Processed(ProcessedInfo),
     SkippedNotChanged,
+    Ignored,
 }
 
 /// The final status of processing an asset
@@ -1185,6 +1184,9 @@ impl ProcessorAssetInfos {
                 // "block until first pass finished" mode
                 info.update_status(ProcessStatus::Processed).await;
             }
+            Ok(ProcessResult::Ignored) => {
+                debug!("Skipping processing (ignored) \"{:?}\"", asset_path);
+            }
             Err(ProcessError::ExtensionRequired) => {
                 // Skip assets without extensions
             }