Add more superscript/subscript symbols

These symbols have certain uses in mathematics. All of them are in `U+207x ~ U+208x` range, and clearly visible in CodeWars editor font.

Proposed key mappings:

```
  ["^+", "⁺"], ["^-", "⁻"], ["^=", "⁼"], ["^(", "⁽"], ["^)", "⁾"],
  ["_+", "₊"], ["_-", "₋"], ["_=", "₌"], ["_(", "₍"], ["_)", "₎"],
```