Get tools service install location from extensions (#16796)

* Get STS install location from extensions

* Update error

extensions/sql-database-projects/src/typings/vscode-mssql.d.ts

@@ -24,6 +24,11 @@ declare module 'vscode-mssql' {
     */
     export interface IExtension {
 
+        /**
+         * Path to the root of the SQL Tools Service folder
+         */
+        readonly sqlToolsServicePath: string;
+
         /**
          * Service for accessing DacFx functionality
          */