Allow all modern CSS units in our stylelint

Problem/Motivation

At the moment, if you want to add something like font-size: 12pt; in a print.css you will fail our stylelint rules since pt is not an allowed value in our set up, even though a 12pt font size is perfectly acceptable for printing.

We should allow all the available, non-deprecated CSS units so stylelint can focus on coding standards rather than what from the spec we can use.

Proposed resolution

Two options (and I'll attach an MR for each):

1. Add all units to the unit-allowed-list array in .stylelint.json
2. Remove the unit-allowed-list parameter so we don't have to update this each time a new unit comes out

Remaining tasks

Add all units to the unit-allowed-list array in .stylelint.json

User interface changes

None

Introduced terminology

None

API changes

None

Data model changes

None