feat(spec): export list of all jsii features (#5021)

Exports a new constant `ALL_SPEC_FEATURES` containing all available jsii extension features. This allows consumers of the `@jsii/spec` package to programmatically access the complete list of features without having to maintain their own copy.

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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

packages/@jsii/spec/src/assembly.ts

@@ -1090,9 +1090,17 @@ export function describeTypeReference(type?: TypeReference): string {
 }
 
 /**
- * Predefined constants for a set of jsii extension features
+ * A list of all jsii extension features
  */
-export type JsiiFeature = 'intersection-types' | 'class-covariant-overrides';
+export const ALL_SPEC_FEATURES = [
+  'intersection-types',
+  'class-covariant-overrides',
+] as const;
+
+/**
+ * Predefined type constants of available jsii extension features
+ */
+export type JsiiFeature = (typeof ALL_SPEC_FEATURES)[number];
 
 /**
  * Determines whether an entity is deprecated.