feat: config option for base

[olamide203]

Description

a base config option that would be used as the default base for log expressions when the log feature gets added in #15 would be nice to have. this would make it possible to configure the base globally. This config option should also have a default value assigned in the TreeNode class.

Use Case

A use case of this would be:

Expression.config({
  angles: 'deg',
  base: 5
});

let x = new Expression('log(2)');
x.evaluate()   // 0.430676558073...

Anything else?

No response