support hot registration emmet (#29986)

* register new included languages in emmet on the fly

* no need to export the function

* only register default completion on startup

* fix tests

extensions/emmet/src/extension.ts

@@ -22,16 +22,7 @@ import { updateExtensionsPath } from 'vscode-emmet-helper';
 import * as path from 'path';
 
 export function activate(context: vscode.ExtensionContext) {
-	let completionProvider = new DefaultCompletionItemProvider();
-	Object.keys(LANGUAGE_MODES).forEach(language => {
-		const provider = vscode.languages.registerCompletionItemProvider(language, completionProvider, ...LANGUAGE_MODES[language]);
-		context.subscriptions.push(provider);
-	});
-	let includedLanguages = getMappingForIncludedLanguages();
-	Object.keys(includedLanguages).forEach(language => {
-		const provider = vscode.languages.registerCompletionItemProvider(language, completionProvider, ...LANGUAGE_MODES[includedLanguages[language]]);
-		context.subscriptions.push(provider);
-	});
+	registerCompletionProviders(context, true);
 
 	context.subscriptions.push(vscode.commands.registerCommand('emmet.wrapWithAbbreviation', (args) => {
 		wrapWithAbbreviation(args);
@@ -137,9 +128,36 @@ export function activate(context: vscode.ExtensionContext) {
 	resolveUpdateExtensionsPath();
 
 	context.subscriptions.push(vscode.workspace.onDidChangeConfiguration(() => {
+		registerCompletionProviders(context, false);
 		resolveUpdateExtensionsPath();
 	}));
 }
 
+/**
+ * Holds any registered completion providers by their language strings
+ */
+const registeredCompletionProviders: string[] = [];
+
+function registerCompletionProviders(context: vscode.ExtensionContext, isFirstStart: boolean) {
+	let completionProvider = new DefaultCompletionItemProvider();
+
+	if (isFirstStart) {
+		Object.keys(LANGUAGE_MODES).forEach(language => {
+			const provider = vscode.languages.registerCompletionItemProvider(language, completionProvider, ...LANGUAGE_MODES[language]);
+			context.subscriptions.push(provider);
+		});
+	}
+
+	let includedLanguages = getMappingForIncludedLanguages();
+	Object.keys(includedLanguages).forEach(language => {
+		if (registeredCompletionProviders.includes(language)) {
+			return;
+		}
+		const provider = vscode.languages.registerCompletionItemProvider(language, completionProvider, ...LANGUAGE_MODES[includedLanguages[language]]);
+		context.subscriptions.push(provider);
+		registeredCompletionProviders.push(language);
+	});
+}
+
 export function deactivate() {
 }