JS: fix h$appendToHsString implementation (#24495)

h$appendToHsString needs to wrap its argument in an updatable thunk
to behave like unpackAppendCString#. Otherwise if a SingleEntry thunk is
passed, it is stored as-is in a CONS cell, making the resulting list
impossible to deepseq (forcing the thunk doesn't update the contents of
the CONS cell)!

The added test checks that the optimization kicks in and that
h$appendToHsString works as intended.

Fix #24495