Add alternative Bulgarian Cyrillic glyphs and Serbianc Cyrillic glyphs

While glyphs of Cyrillic characters in most fonts are based on those being used in Russia by default. Bulgaria and Serbia each have their own set of glyphs. Some of the characters in these sets is different from those used in Russia. Some fonts thus comes with opentype features to change from Russian Cyrillic character set to Serbian Cyrillic set or to Bulgarian Cyrillic set. For example, in Monaspace typefaces:

• cv12 enable Bulgarian–specific Cyrillic alternates
• cv13 enable Serbianc–specific Cyrillic alternates

Thus, I think it'd be really helpful if Maple font also contains Bulgarian and Serbian Cyrillic alternates.

Thank you very much in advance for your consideration.