[feat] More powerful calculation functions

[Kayxue]

Why do you think this is a good feature?
If the future exists, I can finish more advanced calculations using only Rustcast, without using any other tool like a web browser.

Describe the solution you'd like
Add an advanced calculation function to the application directly.

What have you been using currently to solve this?
Now I'm using Raycast to do it.

Additional context
How advanced I hope the functions can be?

1. Convert between currencies.
2. Time Calculation (e.g., 5 minutes before now)
3. Mod operator support (e.g., (100 + 200) % 3)

[unsecretised]

Fair. I believe theres a library for doing modulus and a ton of maths on strings and stuff. Might be worth using that.

Meval

[unsecretised]

I used evalexpr and it adds mod operator support and a bunch of other fancy stuff (like sin and cos and tan)

Time calculation seems to be a difficult one so I'll have to check how to get that to work.
Currency conversion would require integration with some API i believe