Change feature flag for SQL kernel to be user preference (#3838)

* Change feature flag for SQL kernel to be user preference

* fix test that was broken

* Tweak package.nls.json to show "(Preview)"

extensions/notebook/package.json

@@ -26,6 +26,11 @@
 					"type": "string",
 					"default": "",
 					"description": "%notebook.pythonPath.description%"
+				},
+				"notebook.sqlKernelEnabled": {
+					"type": "boolean",
+					"default": false,
+					"description": "%notebook.sqlKernelEnabled.description%"
 				}
 			}
 		},

extensions/notebook/package.nls.json

@@ -4,6 +4,7 @@
 	"notebook.configuration.title": "Notebook configuration",
 	"notebook.enabled.description": "Enable viewing notebook files using built-in notebook editor.",
 	"notebook.pythonPath.description": "Local path to python installation used by Notebooks.",
+	"notebook.sqlKernelEnabled.description": "Enable SQL kernel in notebook editor (Preview)",
 	"notebook.command.new": "New Notebook",
 	"notebook.command.open": "Open Notebook"
 }