Rename application per protoc guidelines

[Panini]

as per the plugins guide on protocol buffers

// A plugin executable needs only to be placed somewhere in the path. The
// plugin should be named "protoc-gen-$NAME", and will then be used when the
// flag "--${NAME}_out" is passed to protoc.

With this, to support the --kotlin_out format instead of supplying a custom plugin, the application should be renamed to protoc-gen-kotlin