add p5 example sketches

[ziyuan-linn]

This PR adds a few faceMesh example sketches provided by @MOQN. These examples should help break down the overwhelming amount of keypoints faceMesh provides. These examples should be live on the web editor after merging this PR and running the p5 upload script.

I tested the examples and they all work great! However, the custom shapes example does not seem to draw the lips correctly.

I haven't done a closer investigation, but I wonder if the problem is related to #107.

[gohai]

@ziyuan-linn I believe we encountered the lips issue before in #67 and 5734120.

Google's tf.js-models code has a list of point-pairs between which one ought to draw a line to arrive at the desired contour.
Our data structure isn't so sophisticated - we try to do a good job now returning the points a meaningful order, but we we can't express e.g. interruptions of the line.

If this is necessary one could either switch to an array of an array of points ([[{x: 1, y: 2}, ... ], [{x: 2, y: 3}, ... ], ... ]) - or to a flat array of line segments ([{x1: 1, y1: 1, x2: 2, y2: 2}, ...]).

[shiffman]

These examples are great!

[shiffman]

Merging, we can address the custom shapes example separately, feel free to open an issue for tracking @ziyuan-linn!