Playground- flexible panels

[jamesnw]

Drag to expand/collapse input/output/console panel widths: see design proposal

[jamesnw]

@stacyk or @mirisuzanne This strongly impacts the grid layout, I'd like to talk through it with someone to get it to a place where styling can happen easily.

[mirisuzanne]

At a quick glance, the grid is defined on .sl-c-playground - currently an even 1fr 1fr split. If you provide a custom property on that element using inline styles (something like --input-track-width: <user-set-width-for-input>), we can set the grid to use that value when defined.

Based on sprint planning, I think it's best to pull in Stacy on this if you need more.

[jamesnw]

Moved upstream to sass#1204