Since our last measurements, startup time in Zed had regressed quite
significantly. This was due to several issues:

- We were loading IBMPlex, which we're not really using in the UI.
- Images were being parsed in the foreground, thus blocking the main
thread
- Language models (for semantic index and assistant) were being loaded
in the foreground, thus blocking the main thread
- Interaction with the keychain was blocking the main thread

In addition to these, with this pull request we will now deserialize the
items for a pane in parallel, as opposed to doing so sequentially.

All combined, when running the zed binary directly this brings startup
time from ~350ms to ~210ms on my machine.

Release Notes:

- Improved startup time.