powi/powf simplexpr functions

[ovalkonia]

Description

Yep, powi/powf, why not

Resolves #1226

Usage

In the showcase

Showcase

Additional Notes

Sooo, it's two separate expressions. They can be reduced down to a single one, that acts like powf. I just though it sometimes miiight be useful to have two of them. Just shrink it down to one expression if that's more appropriate

Checklist

• All widgets I've added are correctly documented.
• I added my changes to CHANGELOG.md, if appropriate.
• The documentation in the docs/content/main directory has been adjusted to reflect my changes.
• I used cargo fmt to automatically format all code before committing

[w-lfchen]

lgtm at a glance, but maybe consider adding this to the grammar using operators as described in #1226. (i am currently unable to judge the feasibility of this)

[ovalkonia]

consider adding this to the grammar using operators as described in #1226

I kinda like it more in the form of a function. I mean, if it's more appropriate to implement it the other way, I would. Although, I would prefer for it to be this way

[elkowar]

This seems fine to me, an operator would also work but I don't really see a massive benefit either way. I'll merge this! Thanks for your contribution!