edit predictions: Preview while holding modifier mode (#24316)

This PR adds a new `inline_completions.inline_preview` config which can
be set to `auto` (current behavior) or to `when_holding_modifier`.
When set to the latter, instead of showing edit prediction previews
inline in the buffer, we'll show it in a popover (even when there's no
LSP completion) so your isn't constantly moving as completions arrive.


https://github.com/user-attachments/assets/3615d151-3633-4ee4-98b9-66ee0aa735b8

Release Notes:

- N/A

---------

Co-authored-by: Danilo <danilo@zed.dev>