Hide publish sql project to new azure server behind preview setting (#21248)

* hide publish to new Azure server under preview setting

* try adding additional properties instead of replacing everything in configuration

* cleanup and change to adding configuration properties instead of replacing the whole thing

* add more info in comment

* remove extra line

extensions/sql-database-projects/package.vscode.json

@@ -4,5 +4,17 @@
 	"extensionDependencies": [
 		"ms-mssql.mssql",
 		"ms-mssql.data-workspace-vscode"
-	]
+	],
+	"contributes": {
+		"configuration": [
+			{
+				"properties": {
+					"sqlDatabaseProjects.enablePreviewFeatures": {
+						"type": "boolean",
+						"description": "%sqlDatabaseProjects.enablePreviewFeatures%"
+					}
+				}
+			}
+		]
+	}
 }