Add add_cursor_to_word command

[ryanabx]

Resolves: #14703

This command works like goto_word, except instead of moving the primary cursor to that word, a new cursor is added to the end of that word.

This is a draft because while I like the idea of arbitrarily placing cursors with the keyboard (similar to alt-click with the mouse), I'm not sure this is the best UX for doing so. For example, you can have your cursor in the middle of one word, use add_cursor_to_word, and have a new cursor at the end of another word, which doesn't make much sense.

I'm beginning to wonder if some kind of "stamp cursor" mode should exist where you move a cursor around with standard movements, and place cursors where you want before they all become usable.

Edit: Seems like marks might have been proposed partially for this purpose? #703 (comment)

[the-mikedavis]

Yeah this strikes me as something that should be covered by marks instead. gw is just one of many ways to make a selection on a point of interest. Being able to add selections on textobjects for example would also be useful.