fix(core): Ungroup placeholder css rules to prevent browsers from r…

…emoving all rules

When rules are grouped using the comma operator and one selector is invalid / unknown then the whole group is invalidated[1].
In this case `::-ms-input-placeholder` is unknown to Firefox and Chrome, so the `::placeholder` rule is never applied.
[1]: https://www.w3.org/TR/selectors-3/#grouping

Signed-off-by: Ferdinand Thiessen <opensource@fthiessen.de>
Signed-off-by: nextcloud-command <nextcloud-command@users.noreply.github.com>