assertPassableString = str => {
  typeof str === 'string' || Fail`Expected string ${str}`;
  !ONLY_WELL_FORMED_STRINGS_PASSABLE || assertWellFormedString(str);
}
For now,
if `ONLY_WELL_FORMED_STRINGS_PASSABLE` environment option is `'enabled'`,
then `assertPassableString` is the same as `assertWellFormedString`.
Otherwise `assertPassableString` just asserts that `str` is a string.

Currently, `ONLY_WELL_FORMED_STRINGS_PASSABLE` defaults to `'disabled'`
because we do not yet know the performance impact. Later, if we decide we
can afford it, we'll first change the default to `'enabled'` and ultimately
remove the switch altogether. Be prepared for these changes.

TODO once the switch is removed, simplify `assertPassableString` to
simply be `assertWellFormedString`.

@param { unknown } str
@returns {asserts str is string }