Support a scale parameter for the font size for stone markings.

[anoek]

Hmm something with this messed up the top/sub positioning:

Should look more like this:

[GreenAsJade]

Note: I found another parameter sub_triangle, which doesn't seem to be tested in this board. It's such a teeny tweak in the maths, I'm kinda hoping it just does the right thing if/when needed 😝

If we know the "right context" for this to be tested in (how to actually set it in main.tsx testcase), I'm happy to add it: it's a parameter of the MoveTree interface, and it's also a valid `MARK_TYPE".

[GreenAsJade]

Oh wait, I just saw that for some reason the original "sub" is actually centered in the square. Is that intended?

I imagined a sub would be positioned in a subscript position, so I didn't notice that I'd "broken" that!

[GreenAsJade]

Latest (note that my Mac gives me bold font on stone lettering!)

[anoek]

Awesome, thanks!