feat: Add complex numbers

[sigmasternchen]

First draft for complex number (#33), please take a look.

Changes from the discussion on the issue:

• I added a public reciprocal function since I needed it internally anyway.
• The power function only takes positive integer exponents, because I used De Moivre’s theorem. Accordingly weighted_product also only supports integer weights. If anyone knows a more generic algorithm, please let me know.

Documentation is also still missing, and I didn't do any changes to the main maths module yet.

Please let me know what you think.

I apologize in advance for not being very responsive - I'm currently on vacation and probably won't have a lot of time to work on this.

- Sigma

[NicklasXYZ]

Overall I think it looks pretty good!

I made some improvements to the weighted_x functions in the main math module based on Hayleigh’s feedback, and I think we could align the complex number implementation in the same way to incorporate that feedback as well (e.g. see the math library thread on the Gleam Discord server under “sharing”).

I think it would be great if we could generalize the weighted_product, power, nth_root, and other related functions, to accept real exponents by implementing the logarithm for complex numbers, such that we can compute z^a = exp(a * log(z)), where z is a complex number, a is a real number, and log(z) = log(|z|) + i * arg(z).

For more info see e.g. https://en.wikipedia.org/wiki/Principal_value and/or https://docs.python.org/3/library/cmath.html#power-and-logarithmic-functions.