Themes: Expose elevations as custom CSS props

[simeonoff]

Is your feature request related to a problem? Please describe

Expose all levels of elevation (0..24) as custom CSS props.

  --igx-elevation-0: ...;
  --igx-elevation-1: ...;

Describe the solution you'd like

1. Create a mixin called igx-elevation-vars that include all custom CSS elevation props in a given scope for a given Sass map as produced by the igx-elevations function.

2. Update the igx-elevation function to retrieve an elevation level as either :

• list of box-shadows (when a concrete $elevations map is passed or $igx-legacy-support is set to true);
• CSS variable ref (when a concrete $elevations map is omitted and $igx-legacy-support is set to false);
• if $igx-legacy-support is set to true and the $elevation map is not passed, try to resolve the elevations from the default $elevation map;

Additional context

This feature is a part of #8868