Draft: 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).

Marking as draft because ideally I'd like all canonical diacritical composite characters to respond to this override too, such as ä and ğ which are used in narrow transcription (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 Default Upright:

Sans Default Italic:

Slab Default Upright:

Slab Default Italic:

Compared to Brill Upright:

Compared to Brill Italic: