feat: allow covariant overrides in classes (#2324)

Relaxes the the restriction on covariant overrides as currently
documented here:

https://aws.github.io/jsii/user-guides/lib-author/typescript-restrictions/#covariant-overrides-parameter-list-changes

Use of this new feature is indicated by the `class-covariant-overrides`
feature in the produced jsii assembly.

## What is the change?

It is now allowed to override the _type of readonly class properties_
and the _return type of class methods_ in class inheritance, as long as
the changes are covariant to the superclass. Importantly, we still don't
allow covariant overrides when implementing interfaces.

✅ now possible

```ts
export class Superclass {}
export class Subclass extends Superclass {}

export class SomethingUnspecific {
  public readonly something = new Superclass();
  public returnSomething(): Superclass {
    return new Superclass();
  }
}

// Subclass is covariant to Superclass
export class SomethingSpecific extends SomethingUnspecific {
  public readonly something = new Subclass();
  public returnSomething(): Subclass {
    return new Subclass();
  }
}
```

❌ still prohibited

```ts
export class Superclass {}
export class Subclass extends Superclass {}

export interface ISomething {
  something: Superclass;
  returnSomething(): Superclass;
}

// ❌ covariant changes are still prohibited when implementing an interface
export class SomethingImpl implements ISomething {
  public something: Subclass = new Subclass();
  public returnSomething(): Subclass {
    return new Subclass();
  }
}
```


## Why is this safe now?

We can make these changes now, because C# 9 has added the necessary
support, and this version of C# has been introduced in .NET 5.
In jsii we have now made the necessary change to [increase the target
framework to `net6.0`](aws/jsii#4916).

C# references:

https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/proposals/csharp-9.0/covariant-returns

https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/override

https://learn.microsoft.com/en-us/dotnet/csharp/programming-guide/classes-and-structs/knowing-when-to-use-override-and-new-keywords


---

By submitting this pull request, I confirm that my contribution is made
under the terms of the [Apache 2.0 license].

[Apache 2.0 license]: https://www.apache.org/licenses/LICENSE-2.0

Author: mrgrain

src/assembler.ts

@@ -226,7 +226,7 @@ export class Assembler implements Emitter {
       jsiiVersion,
       bin: this.projectInfo.bin,
       fingerprint: '<TBD>',
-      usedFeatures: this.usedFeatures.size > 0 ? Array.from(this.usedFeatures) : undefined,
+      usedFeatures: _assemblyFeatures(this.usedFeatures), // might be appended later
     };
 
     if (this.deprecatedRemover) {
@@ -249,6 +249,15 @@ export class Assembler implements Emitter {
 
     const validator = new Validator(this.projectInfo, assembly);
     const validationResult = validator.emit();
+
+    // Inject detected features
+    if (validationResult.usedFeatures) {
+      for (const item of validationResult.usedFeatures) {
+        this.usedFeatures.add(item);
+      }
+      assembly.usedFeatures = _assemblyFeatures(this.usedFeatures);
+    }
+
     if (!validationResult.emitSkipped) {
       const zipped = writeAssembly(this.projectInfo.projectRoot, _fingerprint(assembly), {
         compress: this.compressAssembly ?? false,
@@ -269,6 +278,10 @@ export class Assembler implements Emitter {
       this._afterEmit();
     }
 
+    function _assemblyFeatures(usedFeatures: Set<spec.JsiiFeature>): spec.JsiiFeature[] | undefined {
+      return usedFeatures.size > 0 ? Array.from(usedFeatures).sort() : undefined;
+    }
+
     function _loadReadme(this: Assembler) {
       // Search for `README.md` in a case-insensitive way
       const fileName = fs

src/helpers.ts

@@ -28,7 +28,7 @@ export type MultipleSourceFiles = {
 /**
  * Assembly features supported by this compiler
  */
-export const ASSEMBLY_FEATURES_SUPPORTED: spec.JsiiFeature[] = ['intersection-types'];
+export const ASSEMBLY_FEATURES_SUPPORTED: spec.JsiiFeature[] = ['intersection-types', 'class-covariant-overrides'];
 
 /**
  * Compile a piece of source and return the JSII assembly for it