Fix intermittent JSON schema building errors in CI (#687)

This PR drops the usage of `as const` in `supported-php-extensions.ts`
since they don't seem to play well with the `ts-json-schema-generator`
package that generates Blueprint JSON schema from TypeScript types.

Specifically, it seems to be the root cause behind these intermittent
failures only happening in GitHub actions:

```
UnknownNodeError: Unknown node " supportedPHPExtensions" of kind "Identifier"
```

It's unclear to me why `as const` is fine in
`supported-php-versions.ts`. Perhaps it's because that file uses just
lists and not objects?

I also don't get why the failures are only intermittent. Let's further
investigate if and when these errors return.

packages/php-wasm/universal/src/lib/supported-php-extensions.ts

@@ -1,15 +1,14 @@
-export const supportedPHPExtensions = [
+// The extensions list is repeated on purpose to avoid CI errors
+// @see https://github.com/WordPress/wordpress-playground/pull/687
+export type SupportedPHPExtension = 'iconv' | 'mbstring' | 'xml-bundle' | 'gd';
+export const SupportedPHPExtensionsList = [
 	'iconv',
 	'mbstring',
 	'xml-bundle',
 	'gd',
-] as const;
-export type SupportedPHPExtension = (typeof supportedPHPExtensions)[number];
-export const SupportedPHPExtensionsList =
-	supportedPHPExtensions as any as SupportedPHPExtension[];
+];
 
 export const SupportedPHPExtensionBundles = {
-	'kitchen-sink': supportedPHPExtensions,
-} as const;
-export type SupportedPHPExtensionBundle =
-	keyof typeof SupportedPHPExtensionBundles;
+	'kitchen-sink': SupportedPHPExtensionsList,
+};
+export type SupportedPHPExtensionBundle = 'kitchen-sink';

packages/playground/blueprints/src/lib/compile.ts

@@ -4,12 +4,12 @@ import {
 	LatestSupportedPHPVersion,
 	SupportedPHPExtension,
 	SupportedPHPExtensionsList,
-	SupportedPHPExtensionBundle,
 	SupportedPHPExtensionBundles,
 	SupportedPHPVersion,
 	SupportedPHPVersions,
 	UniversalPHP,
 } from '@php-wasm/universal';
+import type { SupportedPHPExtensionBundle } from '@php-wasm/universal';
 import { isFileReference, Resource } from './resources';
 import { Step, StepDefinition } from './steps';
 import * as stepHandlers from './steps/handlers';
@@ -235,14 +235,14 @@ function compilePHPExtensions(
 ): SupportedPHPExtension[] {
 	const extensions = SupportedPHPExtensionsList.filter((extension) =>
 		requestedExtensions.includes(extension)
-	);
+	) as SupportedPHPExtension[];
 	const extensionsFromBundles = requestedBundles.flatMap((bundle) =>
 		bundle in SupportedPHPExtensionBundles
 			? SupportedPHPExtensionBundles[
 					bundle as SupportedPHPExtensionBundle
 			  ]
 			: []
-	);
+	) as SupportedPHPExtension[];
 	// Deduplicate
 	return Array.from(new Set([...extensions, ...extensionsFromBundles]));
 }