Add global Fs instance (#15044)

This PR adds a global `Fs` instance to the app context.

This will make it easier to access the filesystem in some cases instead
of having to thread it around.

Release Notes:

- N/A

crates/zed/src/main.rs

@@ -14,7 +14,7 @@ use collab_ui::channel_view::ChannelView;
 use db::kvp::KEY_VALUE_STORE;
 use editor::Editor;
 use env_logger::Builder;
-use fs::RealFs;
+use fs::{Fs, RealFs};
 use futures::{future, StreamExt};
 use git::GitHostingProviderRegistry;
 use gpui::{
@@ -399,6 +399,8 @@ fn main() {
             AppCommitSha::set_global(AppCommitSha(build_sha.into()), cx);
         }
 
+        <dyn Fs>::set_global(fs.clone(), cx);
+
         GitHostingProviderRegistry::set_global(git_hosting_provider_registry, cx);
         git_hosting_providers::init(cx);