Motivated by the discussion in helix-editor#10995. Sequential traversal is usually
very fast, unless it's Windows, or the files are on a (slow) remotely
mounted FS.

The existing logic of *not* moving injection of file names to a
background thread is preserved: if a directory can be traversed
in 30ms, all the work happens on the same thread sequentially.

To use `WalkParallel`, we need to give up on file name sorting, so
add a file picker option for that (defaults to sorting enabled to
not change the current behaviour). We _could_ make sorting work
for parallel traversal as well, but that requires eagerly traversing
the file tree and fully realizing the collection of file paths
in memory (so that we can merge partial results from independent
workers). It looks like lazy evaluation is preferred and allows
us to stop the work early (e.g. if the file picker window is closed).

Closes helix-editor#11021.