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/models/cellMagicMapper.ts

@@ -0,0 +1,55 @@
+
+/*---------------------------------------------------------------------------------------------
+*  Copyright (c) Microsoft Corporation. All rights reserved.
+*  Licensed under the Source EULA. See License.txt in the project root for license information.
+*--------------------------------------------------------------------------------------------*/
+
+'use strict';
+
+import { ICellMagicMapper, ILanguageMagic } from 'sql/parts/notebook/models/modelInterfaces';
+
+const defaultKernel = '*';
+export class CellMagicMapper implements ICellMagicMapper {
+	private kernelToMagicMap = new Map<string,ILanguageMagic[]>();
+
+	constructor(languageMagics: ILanguageMagic[]) {
+		if (languageMagics) {
+			for (let magic of languageMagics) {
+				if (!magic.kernels || magic.kernels.length === 0) {
+					this.addKernelMapping(defaultKernel, magic);
+				}
+				if (magic.kernels) {
+					for (let kernel of magic.kernels) {
+						this.addKernelMapping(kernel.toLowerCase(), magic);
+					}
+				}
+			}
+		}
+	}
+
+	private addKernelMapping(kernelId: string, magic: ILanguageMagic): void {
+		let magics = this.kernelToMagicMap.get(kernelId) || [];
+		magics.push(magic);
+		this.kernelToMagicMap.set(kernelId, magics);
+	}
+
+	private findMagicForKernel(searchText: string, kernelId: string): ILanguageMagic | undefined {
+		if (kernelId === undefined || !searchText) {
+			return undefined;
+		}
+		searchText = searchText.toLowerCase();
+		let kernelMagics = this.kernelToMagicMap.get(kernelId) || [];
+		if (kernelMagics) {
+			return kernelMagics.find(m => m.magic.toLowerCase() === searchText);
+		}
+		return undefined;
+	}
+
+	toLanguageMagic(magic: string, kernelId: string): ILanguageMagic {
+		let languageMagic = this.findMagicForKernel(magic, kernelId.toLowerCase());
+		if (!languageMagic) {
+			languageMagic = this.findMagicForKernel(magic, defaultKernel);
+		}
+		return languageMagic;
+	}
+}