When resuming execution of a suspended function, we must not overwrite
any cached `this` value with something from the ExecutionContext.
This was causing an empty JS::Value to leak into the VM when resuming
an async arrow function, since the "this mode" for such functions is
lexical and thus ExecutionContext will have an empty `this`.
It became a problem due to the bytecode optimization where we allow
ourselves to assume that `this` remains cached after we've executed a
ResolveThisBinding in this (or the first) basic block of the executable.
Fixes https://github.com/LadybirdBrowser/ladybird/issues/138
