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)

src/sql/parts/notebook/notebookUtils.ts

@@ -104,4 +104,15 @@ export interface IStandardKernelWithProvider {
 	readonly name: string;
 	readonly connectionProviderIds: string[];
 	readonly notebookProvider: string;
+}
+
+export function tryMatchCellMagic(input: string): string {
+	if (!input) {
+		return input;
+	}
+	let firstLine = input.trimLeft();
+	let magicRegex = /^%%(\w+)/g;
+	let match = magicRegex.exec(firstLine);
+	let magicName = match && match[1];
+	return magicName;
 }