This PR allows legacy decorators to work with strict class fields, which
are now stage 3 and have shipped in a number of browsers. Allowing this will
allow users of the legacy transform (which is currently recommended by the
champions of the decorator proposal) to use the proper class field semantics
for non-decorated fields, which should help prevent breakage later on.
This change is not a breaking change, since users had to explicitly opt into
loose mode in class fields before. This just gives them the option to remove
that opt-in.
