patch 9.0.1948: Vim9: object variable "this." should only be used in constructor

Problem:  Vim9: object variable "this." should only be used in
          constructor
Solution: Disallow to this in normal object methods (other than
          constructors)

closes: #13152
closes: #13212

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: h-east <h.east.727@gmail.com>

runtime/doc/tags

@@ -4483,6 +4483,7 @@ E1387	vim9class.txt	/*E1387*
 E1388	vim9class.txt	/*E1388*
 E1389	vim9class.txt	/*E1389*
 E139	message.txt	/*E139*
+E1390	vim9class.txt	/*E1390*
 E140	message.txt	/*E140*
 E1400	builtin.txt	/*E1400*
 E1401	builtin.txt	/*E1401*

runtime/doc/vim9class.txt

@@ -216,7 +216,8 @@ see this pattern: >
 	      this.col = col
 	   enddef
 	 endclass
-
+<
+							*E1390*
 Not only is this text you need to write, it also has the type of each
 variables twice.  Since this is so common a shorter way to write new() is
 provided: >