Fix the SCSS global variable warnings

[madoar]

This PR fixes the warnings:

DEPRECATION WARNING: As of Dart Sass 2.0.0, !global assignments won't be able to
declare new variables. Consider adding `$aw-param-indicator-width: null` at the root of the
stylesheet.

    ╷
273 │   $aw-param-indicator-width: $width !global;
    │   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    ╵
    stdin 273:3   aw-define-style()
    stdin 393:13  aw-define-styles()
    stdin 600:1   root stylesheet

DEPRECATION WARNING: As of Dart Sass 2.0.0, !global assignments won't be able to
declare new variables. Consider adding `$aw-param-indicator-height: null` at the root of the
stylesheet.

    ╷
274 │   $aw-param-indicator-height: $height !global;
    │   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    ╵
    stdin 274:3   aw-define-style()
    stdin 393:13  aw-define-styles()
    stdin 600:1   root stylesheet

DEPRECATION WARNING: As of Dart Sass 2.0.0, !global assignments won't be able to
declare new variables. Consider adding `$aw-param-indicator-border-width: null` at the root of the
stylesheet.

    ╷
275 │   $aw-param-indicator-border-width: $border-width !global;
    │   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    ╵
    stdin 275:3   aw-define-style()
    stdin 393:13  aw-define-styles()
    stdin 600:1   root stylesheet

See also #268 (comment)

[gethari]

Can this fix be implemented for people who are using the package for Angular 8 as well ?

[madoar]

It kind of depends on #280. Creating a new release is a lot of work, independent of whether it is a bugfix, minor or major release. This PR simply solves a warning. It is kind of a cosmetic change so I wouldn't want to put in the required effort just to create a new bugfix release (5.0.1) for this PR. If #280 gets solved then it would be easy to apply this PR to a 5.x.y branch which can be based on the fix for #280 as well.