Enhance Russian Braille support with mathematical symbols and rules

[Kostenkov-2021]

This PR adds fixes and expands Russian Braille support for MathCAT.

[NSoiffer]

There is a much better way to handle the long if...elseif...elseif....

I was surprised I didn't see any example in a braille file, but there is in an english file. In definitions.yaml, you want to add something like:

- SIPrefixes: {
    "Q": "quetta", "R": "ronna", "Y": "yotta", "Z": "zetta", "E": "exa", "P": "peta", "T": "tera", "G": "giga", "M": "mega", "k": "kilo", "h": "hecto", "da": "deka",
    "d": "deci", "c": "centi", "m": "milli", "µ": "micro", "n": "nano", "p": "pico", "f": "femto", "a": "atto", "z": "zepto", "y": "yocto", "r": "ronto", "q": "quecto"
  }

but in this case, you want the braille. So something like

- TrigFunctions: {
}   "arcsin": "⠫⠁⠎#", ...

and in the rule file you use

"DefinitionValue(. 'Braille', 'TrigFunctions')

FYI: unless Russian braille is very simple, I think you will need emulate some of the other braille files and use "L" for letters, "N" for digits, and a few other things so that after the rules are applied, you can do some cleanup. For example, if there is a rule that a letter after a digit is allowed accept for a-j which are used for digits for 0-9 in Western braille codes, you can adjust the code in the final cleanup. Without the "L" and "N", there is no way to detect that case.