Add Protobuf syntax highlighting

[bernardnormier]

Should be just like slice and csharp, e.g.

syntax = "proto3";

package visitor_center;
option csharp_namespace = "VisitorCenter";

// The protobuf equivalent of Slice interface Greeter in the Greeter example.
service Greeter {
    // Creates a personalized greeting.
    rpc Greet (GreetRequest) returns (GreetResponse);
}

// The request contains the name of the person to greet.
message GreetRequest {
    string name = 1;
}

// The response contains the greeting.
message GreetResponse {
    string greeting = 1;
}

[ReeceHumphreys]

Looks I already added this but the default language keyword is protobuf. I can update it to also support the proto keyword.

[ReeceHumphreys]

Closed by https://github.com/icerpc/icerpc-docs/commit/b1c6c1e8fb0106574c1f2bb005ccfd1df6ca407d