Closes #2723

You can now edit vim.js and set enableEscKeymap to true and tweak
any related settings that you see fit and have a two-character
sequence to ESC from insert mode.

Also updated select block to return selectionStart.
Operators from now on will stop using curStart and curEnd, and rely on the current selection instead. The selection will be generated in evalInput if needed.

Remove off-by-one in vim when head is further forward in the document than the anchor, where vim's selection becomes 1 less than CodeMirror's. This was originally done because the cursor showed past the anchor and looked like an additional character selection.

This reverts commit 4b047883.