add linear gradient support

[sambauers]

• no build files included in this commit
• should work on shapes, text, charts, and tables
• only a basic demo on shapes included

Example usage:

slide.addShape(pptx.shapes.RECTANGLE, {
    x: 5.1,
    y: 6,
    w: 3.0,
    h: 1,
    fill: {
        type: "linearGradient",
        stops: [
            { position: 0, color: '000000', transparency: 10 },
            { position: 100, color: '333333', transparency: 50 },
        ],
        angle: 45,
        scaled: 1,
        rotWithShape: false,
        tileRect: { t: 0, r: 0.5, b: 0.25, l: 1 },
        flip: 'xy',
    },
 });

[w2048185182]

给大佬说666，初步使用状况正常。

[niels-bosman]

@gitbrent Could you have a look at this?