Fix #4089 Linked account cancel (#5347)

VSCode serialization changed rejection handling to only serialize errors, which caused things to break
- Changed to return either account info or a cancel message in the resolve
- Rewrote to use promises. Tracking how to return canceled through 4+ thenables was way trickier than just using a promise
- Updated unit tests to handle new scenario
- Tested integration tests, realized they a) didn't run and b) didn't passed. 
  - Added vscode dev dependency to fix run issue
  - Fixed tests to account for behavior changes in tree state.

src/sql/azdata.proposed.d.ts

@@ -2327,11 +2327,11 @@ declare module 'azdata' {
 	 * AccountProvider.refresh or AccountProvider.prompt are rejected with this error, the error
 	 * will not be reported to the user.
 	 */
-	export interface UserCancelledSignInError extends Error {
+	export interface PromptFailedResult {
 		/**
 		 * Type guard for differentiating user cancelled sign in errors from other errors
 		 */
-		userCancelledSignIn: boolean;
+		canceled: boolean;
 	}
 
 	/**
@@ -2382,7 +2382,7 @@ declare module 'azdata' {
 		 * Prompts the user to enter account information.
 		 * Returns an error if the user canceled the operation.
 		 */
-		prompt(): Thenable<Account>;
+		prompt(): Thenable<Account | PromptFailedResult>;
 
 		/**
 		 * Refreshes a stale account.
@@ -2390,7 +2390,7 @@ declare module 'azdata' {
 		 * Otherwise, returns a new updated account instance.
 		 * @param account - An account.
 		 */
-		refresh(account: Account): Thenable<Account>;
+		refresh(account: Account): Thenable<Account | PromptFailedResult>;
 
 		/**
 		 * Clears sensitive information for an account. To be called when account is removed