Add <is the pen down?> block

[ZenithRogue]

I was playing around in the lib.min.js file in the gui, and it only took me a few lines of code to include a <is the pen down?> block to the pen extension. This has been a long requested feature on the Scratch forums, and is incredibly easy to implement.

I haven't found any issues with it so far, and works correctly across all sprites AFAIK

Block definition:

{
    opcode: "isDown",
    text: l({
        id: "pen.isDown",
        default: "is the pen down?",
        description: "says if the pen is down"
    }),
    blockType: i.BOOLEAN
},

Block function:

{
    key: "isDown",
    value: function(e, t) {
        var A = t.target,
            n = this._getPenState(A);
        return n.penDown
    }
},

I'll open a PR to the extension src/extensions/scratch3_pen/index.js file in a few