LibJS: Get initial_value from local variables if id represents a local

If identifier represents local variable we should get its value from
`local_variables` in `ExecutionContext` instead of environment.

Userland/Libraries/LibJS/Tests/functions/function-default-parameters.js

@@ -151,3 +151,12 @@ test("use variable as default function parameter", () => {
 
     expect(func()).toBe(a);
 });
+
+test("variable is initialized to the value of the parameter if one with the same name exists", () => {
+    function func(a = 1) {
+        var a;
+        return a;
+    }
+
+    expect(func()).toBe(1);
+});