Make mssql extension a module (#18804)

* Rebase from main branch

* import from module

* Add mssql module ref

Co-authored-by: Charles Gagnon <chgagnon@microsoft.com>

extensions/sql-migration/src/typings/ref.d.ts

@@ -8,4 +8,5 @@
 /// <reference path='../../../../src/sql/azdata.proposed.d.ts'/>
 /// <reference path='../../../azurecore/src/azurecore.d.ts'/>
 /// <reference path='../../../azurecore/src/azureResource/azure-resource.d.ts'/>
+/// <reference path='../../../mssql/src/mssql.d.ts'/>
 /// <reference types='@types/node'/>