fix: markdownString must implements MarkupContent

Author: weirongxu

src/edit/textEdit.ts

@@ -14,8 +14,8 @@ export enum EndOfLine {
 
 export enum EnvironmentVariableMutatorType {
   Replace = 1,
-    Append = 2,
-    Prepend = 3
+  Append = 2,
+  Prepend = 3
 }
 
 export class TextEdit implements ITextEdit {

src/markdown/markdownString.ts

@@ -2,10 +2,13 @@
  *  Copyright (c) Microsoft Corporation. All rights reserved.
  *  Licensed under the MIT License. See License.txt in the project root for license information.
  *--------------------------------------------------------------------------------------------*/
+import { MarkupContent, MarkupKind } from "vscode-languageserver-protocol"
 import { URI } from "vscode-uri"
 import { BaseMarkdownString } from "./baseMarkdownString"
 
-export class MarkdownString  {
+export class MarkdownString implements MarkupContent {
+  public kind: MarkupKind = MarkupKind.Markdown
+
   readonly #delegate: BaseMarkdownString
 
   public static isMarkdownString(thing: any): thing is MarkdownString {