jscript: Implement fdexNameCaseInsensitive flag handling.

jscript: Implement fdexNameCaseInsensitive flag handling.

Despite common sense, native doesn't seem to look for exact match first;
it simply case-insensitively compares the props and returns as soon as it
finds one. This is also reliant on implementation details in case the object
has multiple props with same case-insensitive names, e.g. an object having
`Foo` prop, with `foo` prop on its prototype, can still find `Foo` even if
you look up `foo` instead (which matches exactly on the prototype). Which
is not always reliable, sometimes it finds the prototype first.

Signed-off-by: Gabriel Ivăncescu <gabrielopcode@gmail.com>