scanner: add an error for unfinished string literals (#6519)

vlib/v/checker/tests/unfinished_string.out

@@ -0,0 +1,2 @@
+vlib/v/checker/tests/unfinished_string.vv:2:1: error: unfinished string literal
+    1 | a := '

vlib/v/checker/tests/unfinished_string.vv

@@ -0,0 +1 @@
+a := '

vlib/v/scanner/scanner.v

@@ -1193,7 +1193,7 @@ fn (mut s Scanner) ident_string() string {
 	for {
 		s.pos++
 		if s.pos >= s.text.len {
-			break
+			s.error('unfinished string literal')
 		}
 		c := s.text[s.pos]
 		prevc := s.text[s.pos - 1]