Fix Vim 'e' Behavior When Boundary Is Last Point on Line (#7424)

This was originally just to fix
https://github.com/zed-industries/zed/issues/4354, which I did by just
returning the previous offset in `find_boundary`.. but `find_boundary`
is used in the "insert mode" / normal editor too, so returning the
previous boundary breaks existing functionality in that case.

I was considering a new `find_boundary` function just for some of the
vim motions like this, but I thought that this is straightforward enough
and future Vim functions might need similar logic too.

Release Notes:

- Fixed https://github.com/zed-industries/zed/issues/4354

crates/vim/test_data/test_next_word_end_newline_last_char.json

@@ -0,0 +1,3 @@
+{"Put":{"state":"something(ˇfoo)"}}
+{"Key":"}"}
+{"Get":{"state":"something(fooˇ)","mode":"Normal"}}