Add support for P4 Language

Language
P4, Programming Protocol-independent Packet Processors, https://p4.org
P4(Programming Protocol-independent Packet Processors) is a programming language for controlling packet forwarding planes in networking devices, such as routers and switches. In contrast to a general purpose language such as C or Python, P4 is a domain-specific language with a number of constructs optimized for network data forwarding. P4 is distributed as open-source, permissively licensed code, and is maintained by the P4 Project (formerly the P4 Language Consortium), a not-for-profit organization hosted by the Open Networking Foundation.

Additional resources
Just like the P4 language syntax highlighting provided by Github:

#include <core.p4>

typedef bit<16> etherType_t;
typedef bit<48> macAddr_t;

const etherType_t TYPE_IPV4 = 16w0x0800;

header ethernet_t {
    macAddr_t   dstAddr;
    macAddr_t   srcAddr;
    etherType_t etherType;
}

struct headers {
    ethernet_t ethernet;
}

register<bit<32>, bit<1>>(8) statistical_data;

action drop() {
    mark_to_drop(standard_metadata);
}

table ipv4_lpm {
    key = {
        hdr.ipv4.dstAddr: lpm;
    }
    actions = {
        ipv4_forward;
        drop;
        NoAction;
    }
    size = 1024;
    default_action = NoAction();
}

// ...

No file, test, or entry point is named in the issue. Review Prism's existing language definitions and contribution guidance, then use the supplied P4 example and the p4.org link to define the scope; done means P4 code receives syntax highlighting consistent with the requested support.