feat(formats): Add outputReferences support to scss/map-deep

[notlee]

Use the formattedVariables function within the scss/map-deep formatter
to add support for the outputReferences option.
Related issue: #712

before:

$color-core-neutral-0: #ffffff !default;
$color-background-primary: #ffffff !default;

$design-system-tokens: (
  'color': (
    'background': (
      'primary': $color-background-primary
    ),
    'neutral': (
      '0': $color-core-neutral-0,
    )
  )
)

after:

$color-core-neutral-0: #ffffff !default;
$color-background-primary: $color-core-neutral-0 !default;

$design-system-tokens: (
  'color': (
    'background': (
      'primary': $color-background-primary
    ),
    'neutral': (
      '0': $color-core-neutral-0,
    )
  )
)

A side effect of this change is that scss/map-deep also
supports the themeable token property. For backward compatibility
changes have been made so tokens default to being themeable
with scss/map-deep, unlike for scss/variables where tokens
are not themeable by default. See #474

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[notlee]

I'm using an internal themeable_default argument here to maintain the default behaviour of scss/map-deep but we could also pass through a themeable option on the formatter if we wanted.

[dbanksdesign]

This PR is great! Just 2 minor comments about the themeable_default and snapshots and I think we are good!

[dbanksdesign]

LGTM! Thanks for making those changes!