Matrix Controller, styles API

[WildYellowfin]

This PR introduces a matrix controller:

Screen.Recording.2023-04-11.at.16.54.17.mov

This panel can be resized using the --environment-panel-height setting. Aspect ratio is 2:1 so no width is necessary.
The left hand side of the controller is for moving around and manipulating arrays. I plan to make a moveCell(direction) command so that the arrows always move selection to the appropriate cell.

In building this I needed to better type the Environment, so there are now MatrixEnvironment, CasesEnvironment and TabularEnvironment types, as well as type guards. These distinctions govern what shows up in the env panel when it comes to delimiters.

This PR also introduces a "selectionStyle" at window.mathVirtualKeyboard.style. This returns the intersection of styles for all selected atoms. An example use case (mine :) is displaying a rich text editor toolbar with styles information about the selection.

[arnog]

I'm attempting a merge. I assume that

in get selectionStyle(): Style

for (const [key, value] of Object.entries(style)
should be
for (const [key, value] of Object.entries(a.style)

[arnog]

The merge seems to work...

FYI, now typing ( alt/option+Tab creates a pmatrix.

[WildYellowfin]

Closes #600