[Browser] Do not use the Buffer class in web browsers (#2469)

Ensures the WritablePolyfill class uses TextEncoder instead of the
Buffer class when used in web browser context.

Before this PR, triggering calling write() in the browser resulted in a
`Buffer is not defined` error.

 ## Testing instructions

Run this Blueprint locally and confirm it completes successfully

```json
{
  "$schema": "https://playground.wordpress.net/blueprint-schema.json",
  "landingPage": "/wp-admin/admin.php?page=wc-admin",
  "login": true,
  "features": {
    "networking": true,
    "intl": true
  },
  "steps": [
    {
      "step": "installPlugin",
      "pluginData": {
        "resource": "url",
        "url": "https://github-proxy.com/proxy/?repo=woocommerce/woocommerce&release=latest&asset=woocommerce.zip"
      }
    },
    {
      "step": "installTheme",
      "themeData": {
        "resource": "url",
        "url": "https://github-proxy.com/proxy/?repo=woocommerce/woo-themes&branch=trunk&directory=purple"
      }
    },
    {
      "step": "writeFile",
      "path": "/wordpress/wp-content/mu-plugins/rewrite.php",
      "data": "<?php /* Use pretty permalinks */ add_action( 'after_setup_theme', function() { global $wp_rewrite; $wp_rewrite->set_permalink_structure('/%postname%/'); $wp_rewrite->flush_rules(); } );"
    },
    {
      "step": "setSiteOptions",
      "options": {
        "blogname": "WooCommerce",
        "woocommerce_store_city": "Los Angeles",
        "woocommerce_store_address": "123 Main St",
        "woocommerce_store_postcode": "90038",
        "woocommerce_default_country": "United States",
        "woocommerce_onboarding_profile": {
          "skipped": true
        },
        "woocommerce_currency": "USD",
        "woocommerce_weight_unit": "lbs",
        "woocommerce_dimension_unit": "in",
        "woocommerce_allow_tracking": "no",
        "woocommerce_cheque_settings": {
          "enabled": "yes"
        },
        "woocommerce_coming_soon": "no"
      }
    },
    {
      "step": "installPlugin",
      "pluginData": {
        "resource": "url",
        "url": "https://github-proxy.com/proxy/?repo=woocommerce/wc-smooth-generator&release=latest&asset=wc-smooth-generator.zip"
      },
      "options": {
        "activate": true,
        "targetFolderName": "wc-smooth-generator"
      }
    },
    {
      "step": "wp-cli",
      "command": "wp wc generate terms product_cat 10 --max-depth=2"
    },
    {
      "step": "wp-cli",
      "command": "wp wc generate products 20"
    }
  ]
}
```

cc @bacoords @fellyph

Author: adamziel

packages/php-wasm/util/src/lib/writable-polyfill.ts

@@ -62,13 +62,30 @@ export class WritablePolyfill extends EventEmitterPolyfill {
 
 		if (this.ended) {
 			const err = new Error('write after end');
-			this.defer(() => cb(err));
+			// We can't call this.defer() directly. If this.defer is
+			// `queueMicrotask`, a `this.defer()` call will pass the
+			// WritablePolyfill instance as `this` argument and cause
+			// the browser to throw an error similar to "Invalid
+			// invocation".
+			const defer = this.defer;
+			defer(() => cb(err));
 			this.emit('error', err);
 			return false;
 		}
 
 		if (this.decodeStrings && typeof chunk === 'string') {
-			chunk = Buffer.from(chunk, encoding as BufferEncoding);
+			if (
+				typeof Buffer !== 'undefined' &&
+				typeof (Buffer as any).from === 'function'
+			) {
+				chunk = Buffer.from(chunk, encoding as BufferEncoding);
+			} else if (typeof TextEncoder !== 'undefined') {
+				chunk = new TextEncoder().encode(chunk);
+			} else {
+				throw new Error(
+					'String chunks are not supported in this environment: Buffer and TextEncoder are unavailable.'
+				);
+			}
 			encoding = 'buffer' as BufferEncoding;
 		}