Big refactor of action handling

Here's a tentative game plan. The goal of this is to reduce the inconsistency and weirdness in the boundary between ModeHandler and actions, particularly when it comes to multiple cursors.

1. Stop returning VimState from actions. It's not immutable, and it's clear that it's being altered. IMO this is just noise.
2. Stop passing Position to actions. This should always be vimState.cursorStopPosition, and it's clearer if this is used directly.
3. Unify exec, execAction, etc. execCount, unless I'm mistaken, should be in ModeHandler.
4. Rename Range to Cursor and add extra data to it (desiredColumn, etc.). Some usages of Range actually do just refer to a range - these can use vscode.Range.

The hope is that after this, actions will have an exec method that can just be called with each cursor, and nobody will really have to worry about the details of what's going on behind the scenes.

Start by tracing ModeHandler, actions, exec/execAction, execCount, Position, and Range usages to understand the current action boundary and multiple-cursor flow. The refactor is done when actions expose a unified exec method per cursor, no longer receive redundant state or position data, and Range is separated from cursor-specific data as described in the plan.