Deprecate the cols and rows properties of Terminal

As cols and rows are read only (i.e. automatically determined by rect
and font size) there is no need for the properties to be exposed.
Instead, users can get the calculated cols and rows using the get_cols()
and get_rows() methods.

test/platform/unix/unix.test.gd

@@ -212,12 +212,12 @@ class TestPTYSize:
 
 			assert_eq(
 				stty_rows,
-				terminal.rows,
+				terminal.get_rows(),
 				"Expected stty to report correct number of rows for layout '%s'" % s
 			)
 			assert_eq(
 				stty_cols,
-				terminal.cols,
+				terminal.get_cols(),
 				"Expected stty to report correct number of columns for layout '%s'" % s
 			)