feat(event): add TreeRendered (#2324)

* add TreeRendered event

* pass bufnr and winnr to TreeRendered event

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Co-authored-by: Alexander Courtis <alex@courtis.org>

doc/nvim-tree-lua.txt

@@ -2235,6 +2235,15 @@ e.g. handler for node renamed: >
                 handler parameters: ~
                   {buf}          `{number} `API buffer handle (buffer number)
 
+- Event.TreeRendered
+                Invoked every time the tree is redrawn. Normally this event
+                happens after |Event.TreeOpen| except that handlers of this
+                one will have access to the tree buffer populated with the
+                final content.
+                handler parameters: ~
+                  {bufnr}          `{number} `API buffer handle (buffer number)
+                  {winnr}          `{number} `API window handle (window number)
+
 |nvim_tree_events_startup|
 
 There are two special startup events in the form of User autocommands: