(GH-618) Add checkForUpdates setting

This commit adds a "terraform.checkForUpdates" setting which stops the extension
checking if any Language Server updates are available.

Note that according to the VSCode guidelines, there should be a "Do not show again"
button in the notification as well.  Future commits may add this.

package.json

@@ -164,6 +164,12 @@
               "default": false
             }
           }
+        },
+        "terraform.checkForUpdates": {
+          "type": "boolean",
+          "default": true,
+
+          "description": "Automatically check for Terraform Language Server Updates."
         }
       }
     },

src/extension.ts

@@ -175,8 +175,8 @@ async function updateLanguageServer() {
 	const delay = 1000 * 60 * 60 * 24;
 	languageServerUpdater.timeout(updateLanguageServer, delay); // check for new updates every 24hrs
 
-	// skip install if a language server binary path is set
-	if (!config('terraform').get('languageServer.pathToBinary')) {
+	// skip install if a language server binary path is set, or update checks are disabled
+	if (!config('terraform').get('languageServer.pathToBinary') && config('terraform').get('checkForUpdates')) {
 		const installer = new LanguageServerInstaller(installPath, reporter);
 		const install = await installer.needsInstall();
 		if (install) {