Introduce a shared tsconfig for extensions

Setup a baseline tsconfig for extensions to extend. This will help make sure thatof our built-in extensions are using the recommended settings for target and so on. it also reduces duplicated code and will make updating tsconfig settings easier

build/gulpfile.extensions.js

@@ -21,6 +21,7 @@ const nlsDev = require('vscode-nls-dev');
 const root = path.dirname(__dirname);
 const commit = util.getVersion(root);
 const plumber = require('gulp-plumber');
+const _ = require('underscore');
 
 const extensionsPath = path.join(path.dirname(__dirname), 'extensions');
 
@@ -35,7 +36,9 @@ const tasks = compilations.map(function (tsconfigFile) {
 	const absolutePath = path.join(extensionsPath, tsconfigFile);
 	const relativeDirname = path.dirname(tsconfigFile);
 
-	const tsOptions = require(absolutePath).compilerOptions;
+	const tsconfig = require(absolutePath);
+	const tsOptions = _.assign({}, tsconfig.compilerOptions, tsconfig.extends ? require(path.join(extensionsPath, relativeDirname, tsconfig.extends)).compilerOptions : {});
+
 	tsOptions.verbose = false;
 	tsOptions.sourceMap = true;
 

extensions/shared.tsconfig.json

+{
+	"compilerOptions": {
+		"target": "es2017",
+		"module": "commonjs",
+		"lib": [
+			"es6",
+			"es2015.promise"
+		],
+		"strict": true,
+		"alwaysStrict": true,
+		"noImplicitAny": true,
+		"noImplicitReturns": true,
+		"noUnusedLocals": true,
+		"noUnusedParameters": true
+	}
+}
\ No newline at end of file

extensions/typescript-language-features/tsconfig.json

 {
+	"extends": "../shared.tsconfig.json",
 	"compilerOptions": {
-		"target": "es2017",
-		"module": "commonjs",
-		"lib": [
-			"es6",
-			"es2015.promise"
-		],
 		"outDir": "./out",
-		"noImplicitAny": true,
-		"noImplicitReturns": true,
-		"noUnusedLocals": true,
-		"noUnusedParameters": true,
-		"strict": true,
-		"alwaysStrict": true,
 		"experimentalDecorators": true
 	},
 	"include": [