✨ Add new calculator example with variable support

[AnuragVasanwala]

This PR adds a new calculator example (examples/calculator2.jison) that supports variable evaluation.

It is based on the original example calculator.jison and adds only following:

// Variable name RegEx
[a-zA-Z_][a-zA-Z0-9_]*    return 'VARIABLE'

...

// Parse variable value
| VARIABLE
    { $$ = yy[yytext]; }