search buffer contents during global search

[pascalkuthe]

Followup to #5639, addresses #5604 (comment)

Currently, helix always queries the FS when performing global search.
However, when previewing and opening a file helix automatically uses the buffer if it's already open.
That means that the search would report incorrect results for unsaved buffers (which lead to crashes prior to #5639).

This PR creates am implementation of std::io::Read for Rope, so we can easily reuse the existing API in grep_searcher.

This change has the nice side effect of avoiding IO for already open buffers for global search. The performance difference will rarely matter in practice, but it's a nice boost when a huge file is already open in helix.
A similar mechanism can be used for #5645 so that files that are changed but unsaved also get reported as changed.

[pascalkuthe]

Closes #4207

[the-mikedavis]

I'm not familiar enough with Rust parallelism to review most of this but I think this looks good. I left some small typo fix comments

[zyklotomic]

Slightly unrelated to the PR, but wanted to check if my understanding of the UnboundedReceiverStream is correct.

In the asynchronous callback show_picker, do we have to worry about the UnboundedReceiverStream not collecting any values (because all the all_matches_sx from earlier have been dropped)? In WalkerBuilder.run(), inside the Box, in the definition of sink, there is a reference to all_matches_sx, so it won't be dropped. Is this why we don't have to worry about the UnboundedReceiverStream closing early?