Add Groovy DSL

[jamierocks]

def EXTRA = new ExtensionKey(String, 'extra')

def mappings = MappingSetDsl.create {
    klass('a') {
        deobf = 'Demo'
        extension EXTRA, 'demo data'
        
        field('g') { deobf = 'name' }
        
        method('h', '(Z)Ljava/lang/String;') {
            deobf = 'getName'
            
            param(0) { deobf = 'example' }
        }
    }
}

For future reference:
This was written in Java, as it makes types easier, and the produced bytecode is much cleaner and efficient.

For historical value, https://gist.github.com/jamierocks/169d03318b8783df1b9a65d81bfbd5e5 is the beginnings of how the Groovy version would have turned out.