Add /math subcommands

[AspectOfJerry]

Describe your idea
Implement a command to return the digits of pi and add other commands

Describe the solution you'd like
Number parameter for the number of digits (1 to 1000).

[AspectOfJerry]

add gcd and lcm

[AspectOfJerry]

Also check if b ≠ 0 in the quadratic command (currently only checking a)

If a = 0 and b ≠ 0 then the equation is linear, not quadratic.

[AspectOfJerry]

solve system of equation