fix(@angular-devkit/build-angular): support string as plugin option in custom postcss plugin config

In certain cases, the plugin option may be a string value, as shown in the example below:

```json
{
 "plugins": {
  "tailwindcss/nesting": "postcss-nesting"
  }
}
```

In certain cases, the plugin option may be a string value, as shown in the example below:

```json
{
 "plugins": {
  "tailwindcss/nesting": "postcss-nesting"
  }
}
```

See: https://tailwindcss.com/docs/using-with-preprocessors#nesting
(cherry picked from commit 6e8d326)

Author: alan-agius4

packages/angular_devkit/build_angular/src/utils/postcss-configuration.ts

@@ -10,11 +10,11 @@ import { readFile, readdir } from 'node:fs/promises';
 import { join } from 'node:path';
 
 export interface PostcssConfiguration {
-  plugins: [name: string, options?: object][];
+  plugins: [name: string, options?: object | string][];
 }
 
 interface RawPostcssConfiguration {
-  plugins?: Record<string, object | boolean> | (string | [string, object])[];
+  plugins?: Record<string, object | boolean | string> | (string | [string, object])[];
 }
 
 const postcssConfigurationFiles: string[] = ['postcss.config.json', '.postcssrc.json'];
@@ -104,7 +104,7 @@ export async function loadPostcssConfiguration(
 
   const config: PostcssConfiguration = { plugins: [] };
   for (const [name, options] of entries) {
-    if (!options || typeof options !== 'object') {
+    if (!options || (typeof options !== 'object' && typeof options !== 'string')) {
       continue;
     }