Add IPPH/APPH localization override for double-storey ASCII a/g.

[jmcwilliams403]

Basically forces double-storey form when under an 'IPPH'/'APPH' locale like und-fonipa or en-fonipa etc. to avoid confusion with Latin Alpha and Script G.

An example of a font that supports this via localization forms is Brill (shown below).

Ideally, I'd like it if all canonical diacritical composite characters were to respond to this override too, such as ä and ğ (the former of which I implemented in a second commit due to its appearance as an overscript, ◌ᷲ), which are both used in narrow transcription (although admittedly Brill doesn't bother with this part, at least not as of its current version).

<span lang='und-fonipa'>
aɑɐɒᶏᶐ<br>
gɡɠꬶ<br>
ꭤ𝼁<br>
ꬰǥ<br>
äğ
</span>

Sans Upright:

Sans Italic:

Slab Upright:

Slab Italic:

Compared to Brill Upright:

Compared to Brill Italic: