Pen refactor

[0SlowPoke0]

Part of #1870

• added feature Tab swaps to dragging the opposite handle (native: teleports the cursor / web: enters pointer lock and draws a fake cursor at the new location)
• added feature If the current segment is closing the shape by connecting to its start point, the handle that's being dragged should be the closing handle, not a fake handle on the opposite side, and all the above shortcuts should work on that anchor/handle group spanning the start/end point (Tab will modify the shape of the first segment while the current/final segment is still in-progress)