Onboard markdown to use shared tsconfig

extensions/markdown-language-features/tsconfig.json

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