Fixes #22158.
Basically reimplements the whole `StringDecoder` with a much more direct
translation (read like one-to-one) of node's current logic. The old
implementation was closer to node's super old impl and it was too hard
to keep the code structure while matching the behavior of their new
logic.
This adds support for UTF-16LE, ascii, and latin1.
This also enables the node_compat test, which now passes without
modification.
