Draft: Add 3Column-Tall layout variants

[rraihansaputra]

Add 3Column-Tall layout variants, based off the 3Column layout but the main pane is split vertically (like Tall splits the screen).

3column-tall.mp4

Requesting comments on:

• Currently I'm implementing it by adding a Stack variant into the 3Column layout. Please let me know if this adds too much complexity.
• The name. Is 3Column Tall descriptive enough?

Draft tasks:

• Modify ThreeColumnLayout to accept and work with mainColumnStack specification
• register the layout on LayoutType.swift
• Add tests regarding the Tall variants
  • add tests verifying split on the main column
  • add more thorough tests using the frame verifier
• Add layout info to README.md