LibJS: Implement parseInt()

Here's a reasonably faithful implementation of ECMAScript 2021 18.2.5.
Some corner cases are not covered, I've left them as FIXME's in the
included unit test.

Also I had to tweak JS::Value::to_i32() to always convert infinity to
zero, which is in accordance with ToInt32 AFAICT.

Libraries/LibJS/Runtime/GlobalObject.h

@@ -82,6 +82,7 @@ private:
     JS_DECLARE_NATIVE_FUNCTION(is_nan);
     JS_DECLARE_NATIVE_FUNCTION(is_finite);
     JS_DECLARE_NATIVE_FUNCTION(parse_float);
+    JS_DECLARE_NATIVE_FUNCTION(parse_int);
 
     NonnullOwnPtr<Console> m_console;