fix: view.width functions may return strings (#3020)

* Fix get_size() function when size is a function return string

* update view.width help

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

lua/nvim-tree/view.lua

@@ -119,7 +119,7 @@ local function get_size(size)
   if type(size) == "number" then
     return size
   elseif type(size) == "function" then
-    return size()
+    return get_size(size())
   end
   local size_as_number = tonumber(size:sub(0, -2))
   local percent_as_decimal = size_as_number / 100