Add character variant for ầ, ề and ồ

[KrasnayaPloshchad]

The three letters ầ, ề and ồ may have an unusual variant that placed grave accent at top right of circumflex. It would be awesome to implement these alternavive glyphs, and make them accessable via style set (i.e. ssXX feature).

See: https://www.bilibili.com/video/BV1E54y1W7x1

[Bolpat]

@KrasnayaPloshchad You could've added that the symbol occurs at 01:23. (The numbers look like a joke, but aren't. It's where ề is in the word niềm in the subtitles.)

[KrasnayaPloshchad]

Before this video, I’ve also seen it from older videos from South Vietnam era.