Part of https://github.com/zed-industries/zed/discussions/31653
`"use_system_path_prompts": false` is needed in settings for these to
appear as modals for new file save and file open.

Fixed a very subpar experience of the "save new file" Zed modal,
compared to a similar "open file path" Zed modal by uniting their code.

Before:


https://github.com/user-attachments/assets/c4082b70-6cdc-4598-a416-d491011c8ac4


After:



https://github.com/user-attachments/assets/21ca672a-ae40-426c-b68f-9efee4f93c8c


Also 

* alters both prompts to start in the current worktree directory, with
the fallback to home directory.
* adjusts the code to handle Windows paths better

Release Notes:

- Improved Zed prompts for file path selection

---------

Co-authored-by: Smit Barmase <heysmitbarmase@gmail.com>