Make fzf lazy-loadable

[dapetcu21]

This only requires the fzf-lua.previewer.builtin module when listing the sessions, which allows fzf to be lazy loaded (with folke/lazy.nvim) only when the list() action is triggered.

Otherwise, autoload = true cannot be used without loading fzf at startup as well.

[dapetcu21]

Nevermind. Apparently lazy loads it anyway somehow. Might re-open after some more testing.

[gennaro-tedesco]

Thank you for looking into this. I remember trying a few such mechanisms back then (loading some modules within function calls rather than at import), however I did not manage to have it working consistently (generally I do not really understand too well how the different plugin managers lazy load dependencies, hence something that may be lazy loading fine with one plugin manager may not necessarily with another one).