feat: report undefined variable errors on the line where they are used

Store the line number of first use in implicitly declared variables as a numeric value, then synthesize a token at that line when reporting the error, ensuring bounded error message length even for pathologically long variable names.

test/language/nonlocal/undefined.wren

@@ -1,6 +1,4 @@
 var fn = Fn.new {
-  System.print(Foo)
-  System.print(Bar)
+  System.print(Foo) // expect error
+  System.print(Bar) // expect error
 }
-
-// expect error line 6