feat(events): add DirChangedPre

In Nvim, like DirChanged, this also triggers when switching windows.

This marks Vim patch 8.2.4335 as ported.

vim-patch:8.2.4335: no autocommand event triggered before changing directory

Problem:    No autocommand event triggered before changing directory. (Ronnie
            Magatti)
Solution:   Add DirChangedPre. (closes vim/vim#9721)
28e8f73ae2

runtime/doc/autocmd.txt

@@ -525,8 +525,19 @@ DirChanged			After the |current-directory| was changed.
 					"global"  to trigger on `:cd`
 					"auto"    to trigger on 'autochdir'.
 				Sets these |v:event| keys:
-				    cwd:   current working directory
-				    scope: "global", "tabpage", "window"
+				    cwd:            current working directory
+				    scope:          "global", "tabpage", "window"
+				    changed_window: v:true if we fired the event
+				                    switching window (or tab)
+				<afile> is set to the new directory name.
+				Non-recursive (event cannot trigger itself).
+							*DirChangedPre*
+DirChangedPre			When the |current-directory| is going to be
+				changed, as with |DirChanged|.
+				The pattern is like with |DirChanged|.
+				Sets these |v:event| keys:
+				    directory:      new working directory
+				    scope:          "global", "tabpage", "window"
 				    changed_window: v:true if we fired the event
 				                    switching window (or tab)
 				<afile> is set to the new directory name.

runtime/doc/vim_diff.txt

@@ -433,7 +433,8 @@ Vimscript compatibility:
   `this_session` does not alias to |v:this_session|
 
 Working directory (Vim implemented some of these later than Nvim):
-- |DirChanged| can be triggered when switching to another window.
+- |DirChanged| and |DirChangedPre| can be triggered when switching to another
+  window or tab.
 - |getcwd()| and |haslocaldir()| may throw errors if the tab page or window
   cannot be found.  *E5000* *E5001* *E5002*
 - |haslocaldir()| checks for tab-local directory if and only if -1 is passed as