add vscode-dts-compile-check to ensure that vscode.d.ts is self-contained, esp that it doesn't depend on vscode.proposed.d.ts, and that uses es5 only

src/tsconfig.vscode-dts.json

@@ -0,0 +1,22 @@
+{
+	"compilerOptions": {
+		"noEmit": true,
+		"module": "None",
+		"experimentalDecorators": false,
+		"noImplicitReturns": true,
+		"noImplicitOverride": true,
+		"noUnusedLocals": true,
+		"allowUnreachableCode": false,
+		"strict": true,
+		"exactOptionalPropertyTypes": false,
+		"useUnknownInCatchVariables": false,
+		"forceConsistentCasingInFileNames": true,
+		"types": [],
+		"lib": [
+			"es5",
+		],
+	},
+	"include": [
+		"vs/vscode.d.ts"
+	]
+}