New SASS variable: $input-select-focus-bg

[pardeyke]

Prerequisites

• I have searched for duplicate or closed feature requests
• I have read the contributing guidelines

Proposal

Add the SASS variable $input-select-focus-bg to control bg of focused selects.

Motivation and context

As $input-focus-bg can tint the bg of focused inputs I think it would also make sense to control the bg of focused select inputs.

[mdo]

Passing per my comment in #42050.