Add the "Isolated" parameter in the *.network file, e.g.,
[Bridge]
Isolated=true|false
When the Isolated parameter is true, traffic coming out of this port
will only be forward to other ports whose Isolated parameter is false.
When Isolated is not specified, the port uses the kernel default
setting (false).
The "Isolated" parameter was introduced in Linux 4.19.
See man bridge(8) for more details.
But even though the kernel and bridge/iproute2 recognize the "Isolated"
parameter, systemd-networkd did not have a way to set it.
Those are all consumed by our parser, so they all support comments.
I was considering whether they should have a license header at all,
but in the end I decided to add it because those files are often created
by copying parts of real unit files. And if the real ones have a license,
then those might as well. It's easier to add it than to make an exception.