feature: new command vi-swapcase-and-forward-char for vi-mode.

[sakurawald]

To close the issue: #1621

The command vi-swapcase-and-forward-char is used as a shortcut to swap-case on cursor and forward char in vi-normal mode.
The vi-swapcase is an vi-operator, which requires an extract motion action, to decide the region of swap-case, which may be an over-kill for some simple-case.

If you only want to swapcase for a consecutive characters, just simply press the ~ key, and this new command will swap the case of character, move the cursor to forward character, repeatly.

[sakurawald]

For some reason, the file is re-formatted by the formatter in my env.
I don't know if this is correct. I am using the default code formatter provided by lem editor.