refactor(sass): separate sass variables from styles

[brandyscarney]

Short description of what this resolves:

Separates sass variables from styles for all components

Changes proposed in this pull request:

• Adds *.var.scss files for all components
• Adds support for avatar as a stand alone component
• Adds avatar test
• Removes deprecated variables

Ionic Version: 4.x

Fixes: closes #9192 closes #12880

[brandyscarney]

Here are the remaining files that include variables but don't end in *.vars.scss:

[ix-xerri]

What is the best way to use mixins in Ionic 4? I would like to have globally and not import the mixins file where they are used. I've tried editing angular.json, global.scss and variables.scss but no luck. Error is always thrown that the mixins are not initialised.