Update List Tree visual design with chevrons

[elioqoshi]

Based on https://github.com/canonical-web-and-design/vanilla-squad/issues/968#issuecomment-823104390 we should update the visuals for list trees as well. They are currently using + and -:

https://vanillaframework.io/docs/patterns/list-tree

[clagom]

WG: we decided to update the + and - icons in squares with the new icons: https://vanillaframework.io/docs/examples/patterns/icons/icons-light chevron within squares.

[lyubomir-popov]

Needs a visual so not ready for dev. Moving to icebox.

[clagom]

WG: moving to dev and eventually QA design wise