Reapply parcel/watcher adoption

Revert "Revert e7fffbf1c9169087f1098aedfe54c59c079fa3ac"

This reverts commit 6786b0ad7d.

Two changes:

- Lazy import parcel/watcher
- Add `@parcel/watcher` as a dev dep in extensions folder so that we pull in correct version for build os

extensions/markdown-language-features/esbuild-preview.js

@@ -2,6 +2,7 @@
  *  Copyright (c) Microsoft Corporation. All rights reserved.
  *  Licensed under the MIT License. See License.txt in the project root for license information.
  *--------------------------------------------------------------------------------------------*/
+// @ts-check
 const path = require('path');
 const esbuild = require('esbuild');
 
@@ -15,20 +16,30 @@ if (outputRootIndex >= 0) {
 	outputRoot = args[outputRootIndex + 1];
 }
 
+const srcDir = path.join(__dirname, 'preview-src');
 const outDir = path.join(outputRoot, 'media');
 
-esbuild.build({
-	entryPoints: [
-		path.join(__dirname, 'preview-src', 'index.ts'),
-		path.join(__dirname, 'preview-src', 'pre'),
-	],
-	bundle: true,
-	minify: true,
-	sourcemap: false,
-	format: 'esm',
-	outdir: outDir,
-	platform: 'browser',
-	target: ['es2020'],
-	watch: isWatch,
-	incremental: isWatch,
-}).catch(() => process.exit(1));
+function build() {
+	return esbuild.build({
+		entryPoints: [
+			path.join(srcDir, 'index.ts'),
+			path.join(srcDir, 'pre'),
+		],
+		bundle: true,
+		minify: true,
+		sourcemap: false,
+		format: 'esm',
+		outdir: outDir,
+		platform: 'browser',
+		target: ['es2020'],
+	});
+}
+
+build().catch(() => process.exit(1));
+
+if (isWatch) {
+	const watcher = require('@parcel/watcher');
+	watcher.subscribe(srcDir, () => {
+		return build();
+	});
+}