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/sqltest/parts/notebook/common.ts

@@ -17,6 +17,7 @@ export class NotebookModelStub implements INotebookModel {
 	constructor(private _languageInfo?: nb.ILanguageInfo) {
 	}
 	public trustedMode: boolean;
+	language: string;
 
 	public get languageInfo(): nb.ILanguageInfo {
 		return this._languageInfo;

src/sqltest/parts/notebook/model/cell.test.ts

@@ -27,7 +27,7 @@ describe('Cell Model', function (): void {
 
 	it('Should update values', async function (): Promise<void> {
 		let cell = factory.createCell(undefined, undefined);
-		cell.language = 'sql';
+		cell.setOverrideLanguage('sql');
 		should(cell.language).equal('sql');
 		cell.source = 'abcd';
 		should(cell.source).equal('abcd');

src/sqltest/parts/notebook/model/notebookModel.test.ts

@@ -96,6 +96,7 @@ describe('notebook model', function(): void {
             connectionService: queryConnectionService.object,
             providerId: 'SQL',
             standardKernels: [{ name: 'SQL', connectionProviderIds: ['MSSQL'], notebookProvider: 'sql' }],
+            cellMagicMapper: undefined,
             defaultKernel: undefined,
             layoutChanged: undefined,
             capabilitiesService: capabilitiesService.object