gpui: Do not use a single shared parker within a Dispatcher (#40417)

This caused issues with #40172, as it made Zed execute and block on tad
few more background tasks. Parker is ~cheap to create, hence we should
be ok to just create it at the time it is needed.

Release Notes:

- N/A

---------

Co-authored-by: Cole Miller <cole@zed.dev>
3780fe3b8ePiotr Osiewicz committed on 10/16/2025, 9:52:09 PM· committed by GitHubparent59991e9
7 files changedLine totals unavailable