We already have an equivalent header within FEXCore that's header only,
so we can adapt it to conform for both cases, allowing for removal of
one of them.
ltrim had the issue that it would always consume the left-most character
even if it wasn't whitespace. So `FEXLoader` would turn in to
`EXLoader`, even without any whitespace in the string.
Adds a test to ensure this doesn't occur again.
LC