[chore] Add several vis state mergers combineConfigs and improve TS

[igorDykhta]

• Adds combineConfigs to some vis state saved configs that are non-array complex objects:

  1. interactionConfig
  2. layerBlending
  3. overlayBlending
  4. editor

  Each of these has inline comments (and unit tests) explaining the decisions made on how to combine values in different situations. For example, interactionConfig.brush.size gets aggregated into keeping the single largest value among all brush sizes.

• Improves and fixes some TS defs.

• Adds jest unit tests for newly added combineConfigs.