Functions and Exponents

I’ve just added support for exponents and standard mathematical
functions, including sin, cos, tan, ln, log, and sqrt.

The implementation was fairly straightforward. The lexer simply
tokenizes the ^ character into a Power token. Extending the lexer
from there was just a matter of adding the token rule and implementing
the corresponding evaluation logic.

The Precedence Gotcha

I did run into one classic mathematical quirk with my parser rules.
Currently, -x^2 is parsed as (-x)^2 because the unary minus operator
mistakenly has higher precedence than the exponent. Standard
mathematical convention dictates it should be evaluated as -(x^2).
While I plan to fix this in the parser proper, the current workaround is
just to use explicit parentheses when writing negative exponents.

Next Up: Symbolic Differentiation

As a final step for this milestone, I want to implement symbolic
differentiation utilizing the chain rule. This will allow the engine
to take simple expressions like x^2 or sin(2x) and automatically
differentiate them into 2*x and 2*cos(2x) respectively.