Sel's "direction" argument is rather unclear. Support more direction names or add them to documentation?

## Describe your suggestion
The arguments to sel are rather vague, especially without examples. Either making it clear that direction needs to be cardinal, or adding support for other methods of giving directions, such as "forward", "left", or x y or z then a positive or negative number, may be helpful.

## Settings
No extra settings would be needed for this.

## Context
I was having trouble figuring out how to make sel expand on the positive x axis. I tried options such `sel expand n x 4` and `sel expand n front 4`, but neither worked, giving the error `Error at argument #4: Expected ForEnumFacing`. I had to eventually dive into the source code to figure out that it was asking for cardinal directions rather than an x y or z axis.

## Final checklist
- [x] I know how to properly use check boxes
- [x] I have not used any OwO's or UwU's in this issue.