Helix: Add support for Trim Whitespace from selection

[jnns]

Body

What are you proposing?

Helix has the following keybinding:

_	Trim whitespace from the selection	trim_selections

Why does this matter?

The action trim_selections removes whitespace left and right from a selected line.
This is useful, for example, after selecting a whole line with x.

[freepicheep]

Wow, I didn't even realize helix had this. Definitely useful and shouldn't be too hard to implement.