[sql mode] charset casting syntax

* add support to "charsetCast" with syntax n str or N str or _charset str
* add example in index.html
[sql mode] Add comments with links to supported syntaxes documentation
[vim keymap] Repeat insert changes for A,I,a,i,caw,ciw
[vim keymap] Fixups for repeat insert mode changes

Cleans a lot of insert mode record/replay code.
Adds support for o and O.
Adds unit tests and fixes a lot of corner cases.
Fixes cw deleting trailing whitespace.
[vim keymap] Add support for insert mode count repeat.
[vim keymap] Rewrite insert mode change recording