Allow DOCTYPE in protocol definition

[Nxllpointer]

Currently wayland-scanner only allows the <?xml ... ?> declaration and the protocol tag at the start of a xml protocol definition. This PR allows one to additionaly specify a <!DOCTYPE ...> like this:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE protocol SYSTEM "https://gitlab.freedesktop.org/wayland/wayland/-/raw/ee12e69b8fb8c55460126e4e527eb4f7fee5526e/protocol/wayland.dtd">
<protocol name="test_proto">
    <interface name="test_interface" version="1">
        <description summary="Testing"></description>
        <event name="ping"></event>
    </interface>
</protocol>

The specified file can be consumed by LSPs to enable assisting with autocompletion and syntax checking.

[Nxllpointer]

Thank you