Visual mode rewrite

I'm rewriting the way that visual mode is written to:
1. Fix some tricky bugs, like https://github.com/codemirror/CodeMirror/issues/2913, and a couple I uncovered while doing this. (Unit tests updated)
2. Significantly simplify blockwise visual mode

The core of the changes here is that I'm creating a vim selection model. The selection model consists of

```
vim.sel.head
vim.sel.anchor
vim.visualMode
vim.visualLine
vim.visualBlock
```

I'm creating a 2-way binding between the vim and codemirror selection models. All motions now rely only on the vim selection model. Operators and actions still rely on the codemirror selection model, since they operate directly on the text.

Sorry for the gigantic change but I couldn't make it any smaller and still pass all the tests... I'm planning to delete `getSelectedAreaRange`, `getLastSelectedAreaRange`, and `selectBlock` in future changes as they are obsolete now that vim keeps its own selection model.

An added bonus here is that visual line mode now has a cursor that can be moved.

@heppe and @nightwing please take a look and see if you can catch any bugs. I'm sure there are some.
add a comment explaining what "sel" is.