[postcss-custom-selectors] : replacement focussed on `importFrom` / `exportTo`

[romainmenke]

see : #527

• importFrom and exportTo have been removed from this plugin.
• a new plugin needs to be created to replace it.
• this new plugin would only do importFrom / exportTo

Although we have the design tokens plugin it is not a drop in replacement and has a much smaller scope. Providing a drop in replacement will cause less issues for users.

[romainmenke]

The drop-in replacements have been created but we will not be releasing this.
We have decided against taking on maintenance for these plugins.

Anyone is free to take the source code for these replacement plugins and release them as their own!

#734