Improve cell language detection and add support for language magics (#4081)

* Move to using notebook language by default, with override in cell
* Update cell language on kernel change
* Tweak language logic so that it prefers code mirror mode, then falls back since this was failing some notebooks
* Add new package.json contribution to define language magics. These result in cell language changing. Language is cleared out on removing the language magic
* Added support for executing Python, R and Java in the SQL Kernel to prove this out. It converts to the sp_execute_external_script format

TODO in future PR:

* Need to hook up completion item support for magics (issue #4078)
* Should add indicator at the bottom of a cell when an alternate language has been detected (issue #4079)
* On executing Python, R or Java, should add some output showing the generated code (issue #4080)

extensions/notebook/package.json

@@ -130,6 +130,26 @@
 				"key": "Ctrl+Shift+T",
 				"when": "notebookEditorVisible"
 			}
+		],
+        "notebook.languagemagics": [
+			{
+				"magic": "lang_python",
+				"language": "python",
+				"executionTarget": null,
+				"kernels": ["sql"]
+			},
+			{
+				"magic": "lang_r",
+				"language": "r",
+				"executionTarget": null,
+				"kernels": ["sql"]
+			},
+			{
+				"magic": "lang_java",
+				"language": "java",
+				"executionTarget": null,
+				"kernels": ["sql"]
+			}
 		]
 	},
 	"dependencies": {