Extensions: drop-down menus

[cwillisf]

Implement drop-down menus for extensions.

For example, the getInfo() snippet below should display a drop-down menu. See the extension spec for more detail, including more advanced examples.

menus: {
    lessMore: ['<', '>']
},
blocks: [
    {
        opcode: 'whenDistance',
        text: 'when distance [OP] [REFERENCE]',
        blockType: BlockType.HAT,
        arguments: {
            OP: {
                type: ArgumentType.STRING,
                menu: 'lessMore',
                defaultValue: '<'
            },
            REFERENCE: {
                type: ArgumentType.NUMBER,
                defaultValue: 50
            }
        }
    }
]