patch 8.2.3314: behavior of exists() in a :def function is unpredictable

Problem:    Behavior of exists() in a :def function is unpredictable.
Solution:   Add exists_compiled().

runtime/doc/usr_41.txt

@@ -1182,6 +1182,7 @@ Various:					*various-functions*
 	state()			get current busy state
 	visualmode()		last visual mode used
 	exists()		check if a variable, function, etc. exists
+	exists_compiled()	like exists() but check at compile time
 	has()			check if a feature is supported in Vim
 	changenr()		return number of most recent change
 	cscope_connection()	check if a cscope connection exists