Add n: modifier to get length of a format, also automatically expand

variable name arguments again if they contain a #{.

tmux.1

@@ -4574,6 +4574,9 @@ pads the string to a given width, for example
 .Ql #{p10:pane_title}
 will result in a width of at least 10 characters.
 A positive width pads on the left, a negative on the right.
+.Ql n
+expands to the length of the variable, for example
+.Ql #{n:window_name} .
 .Pp
 Prefixing a time variable with
 .Ql t:\&