chore: implement suggestions

lib/Compiler.js

@@ -178,6 +178,8 @@ class Compiler {
 		this.root = this;
 		/** @type {string} */
 		this.outputPath = "";
+		/** @type {Watching} */
+		this.watching = undefined;
 
 		/** @type {OutputFileSystem} */
 		this.outputFileSystem = null;

lib/Watching.js

@@ -321,6 +321,7 @@ class Watching {
 			const shutdown = () => {
 				this.compiler.cache.shutdown(err => {
 					this.compiler.hooks.watchClose.call();
+					this.compiler.watching = undefined;
 					const closeCallbacks = this._closeCallbacks;
 					this._closeCallbacks = undefined;
 					for (const cb of closeCallbacks) cb(err);