feat: Make the terminal on tutorials resizable with SplitPane

[bricefrisco]

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Closes #1740 by wrapping the viewer and terminal in a SplitPane.

Quick demo:
screen-capture.webm

Update (see comments): Fixes #624

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[bricefrisco]

Looked into this more, the styling can be simplified because the overflow-y: auto on .terminal is clipping .terminal::after as it is anyways.

It looks like it was meant to create a top fade / depth separator between the terminal and whatever is above it, but that will now be done by SplitPane.

Also no need to set the visible class any longer, that logic is now handled by:

<SplitPane
		min="50px"
		type="vertical"
		disabled={!terminal_visible}
		max={terminal_visible ? '80%' : '100%'}
		pos={terminal_visible ? '20%' : '100%'}
	>

As I was going along, I stumbled upon and decided to quickly fix #624 by increasing the opacity on the background.

Before:

After:

With the terminal in the after photo being resizable as well. I'm pushing those changes now.