Prevent use of optional chaining and nullish coalescing in the src/shared/ folder

Given that this code is used on the worker-thread, where SystemJS is still used during development, we need to (for now) handle this folder the same way as the `src/core/` one.

src/shared/.eslintrc

@@ -0,0 +1,13 @@
+{
+  "parserOptions": {
+    "ecmaVersion": 2017,
+  },
+
+  "extends": [
+    "../.eslintrc"
+  ],
+
+  "env": {
+    "es2017": true,
+  },
+}