Change state shape in preparation for split pane views

[curran]

This set of changes is a fairly large refactoring to introduce a recursive data structure representing split panes.

This is the foundation upon which we can build out the split pane functionality described in #705 .

The main change is introduction of these types:

// PaneId
//   * A unique ID for a pane.
//   * This is a random string.
export type PaneId = string;

// The leaf node of the tree data structure
export type LeafPane = {
  type: 'leafPane';
  // The list of tabs
  // Mutually exclusive with `children`.
  tabList: Array<TabState>;

  // The ID of the file that is currently active
  // within this leaf pane.
  // Invariant: `activeFileId` is always in `tabList`.
  activeFileId: FileId | null;
};

// Internal node of the tree data structure
export type SplitPane = {
  type: 'splitPane';

  // Which orientation is it? Vertical split or horizontal split?
  // Applies only to `children`
  orientation: 'vertical' | 'horizontal';

  // The children panels
  // Mutually exclusive with `tabList`.
  children: Array<Pane>;
};

// The node data structure of the split pane tree
export type Pane = {
  // Every pane gets a unique ID,
  // so we can refer to it in actions.
  id: PaneId;

  // The type of the pane
  // Either a leaf pane or a split pane.
  type: 'leafPane' | 'splitPane';
} & (LeafPane | SplitPane);

then updating the rest of the codebase to match.