settings ui: Update file headers when adding or removing projects (#40968)

This PR gets the `SettingsWindow` struct to subscribe to all
`Entity<Project>` events and any future project entities that are
created. When a project emits an event that signals a worktree has been
added or removed, the settings window refetches all settings files it
can find.

This fixes a bug where the settings ui would notice some project
settings that were created or opened after the `SettingsWindow` has been
initialized.

I also renamed `LOCAL` file mask to `PROJECT` to be inline with the
`SettingsFile` naming convention.

Release Notes:

- settings ui: Fix bug where project setting files wouldn't be detected
if they were created or opened after while an active settings window is
open

crates/workspace/src/workspace.rs

@@ -1458,7 +1458,7 @@ impl Workspace {
             }),
             cx.on_release(move |this, cx| {
                 this.app_state.workspace_store.update(cx, move |store, _| {
-                    store.workspaces.remove(&window_handle.clone());
+                    store.workspaces.remove(&window_handle);
                 })
             }),
         ];