Added `globalThis` property (#495)

* added builtin globalThis

* forgot to initialize globalThis in the builtin mod.rs file

* changed  to  to match naming conventions and fixed issue with suggested edits to globalThis' initial value

* updated the test for the  property as suggested

boa/src/builtins/global_this/mod.rs

@@ -0,0 +1,11 @@
+#[cfg(test)]
+mod tests;
+
+use crate::{builtins::value::Value, BoaProfiler};
+
+/// Initialize the `globalThis` property on the global object.
+#[inline]
+pub fn init(global: &Value) {
+    let _timer = BoaProfiler::global().start_event("globalThis", "init");
+    global.set_field("globalThis", global.clone());
+}

boa/src/builtins/global_this/tests.rs

@@ -0,0 +1,10 @@
+use crate::exec;
+
+#[test]
+fn global_this_exists_on_global_object_and_evaluates_to_an_object() {
+    let scenario = r#"
+        typeof globalThis;
+        "#;
+
+    assert_eq!(&exec(scenario), "object");
+}

boa/src/builtins/mod.rs

@@ -6,6 +6,7 @@ pub mod boolean;
 pub mod console;
 pub mod error;
 pub mod function;
+pub mod global_this;
 pub mod json;
 pub mod math;
 pub mod nan;
@@ -35,6 +36,7 @@ pub fn init(global: &Value) {
     Array::init(global);
     BigInt::init(global);
     Boolean::init(global);
+    global_this::init(global);
     json::init(global);
     math::init(global);
     nan::init(global);