Allow script a/g to access all single-storey variants of a/g.

[jmcwilliams403]

This effectively makes them a true forced-single-storey version of a/g, using the auto-serifed mechanism I set up for instances where the selected a/g variant is double-storey.

On a side note, it would be really helpful and would clean things up a lot if it were possible to check for whether the font was slab from within variants.toml at compile time.

aɑɐɒᶏᶐ
gɡɠꬶ
ꭤ𝼁

Sans Upright:

Sans Italic:

Slab Upright:

Slab Italic:

SS03 Sans Upright:

SS03 Sans Italic:

SS03 Slab Upright:

SS03 Slab Italic:

SS15 Sans Upright:

SS15 Sans Italic:

SS15 Slab Upright:

SS15 Slab Italic: