Add more size modifiers to modal component

[Sebastian Nitu]

Problem

The difference between the default and modal_size_sm modifiers is a bit too drastic and would make sense to set the current modal_size_sm to actually be modal_size_xs and add a new value for sm that's in-between the default and xs. Maybe it's worth creating a map for the output?

Example map:

$modal-size-scale: (
  'xs': 18em,
  'sm': 26em,
  'md': 38em,
  'lg': 46em,
  'xl': 58em
);