zsh keybindings: ctrl-t: pass the last buffer word as query

[bretello]

When using the Ctrl-T keybinding, to complete paths, I often start writing part of a path only to realize I want to use fzf.

With this patch, Ctrl-T now uses the last word in the buffer as initial query for fzf, which improves this scenario.

Here's a demo:

This always uses the last word in the buffer, so typing something like ls<space><ctrl-t> will start fzf with ls as query

[bretello]

looking into the test failures

[tmpm697]

You achieve this with shell functions.

[eli-percepto]

Consider the ideas from here: #3918