scanner: fix string slash line breaks error

vlib/builtin/string_test.v

@@ -728,3 +728,14 @@ fn test_split_into_lines() {
 		assert line == line_content
 	}
 }
+
+fn test_string_literal_with_backslash(){
+    a := 'Hello\
+          World'
+    assert a == 'HelloWorld'
+
+	b := 'One\
+		  Two\
+		  Three'
+	assert b == 'OneTwoThree'
+}

vlib/v/scanner/scanner.v

@@ -900,13 +900,31 @@ fn (s mut Scanner) ident_string() string {
 	if s.is_inside_string {
 		end++
 	}
-	if start > s.pos {}
-	else {
+	if start <= s.pos {
+		if s.text[start..end].contains('\\\n') {
+			lit = trim_slash_line_break(s.text[start..end])
+		} else {
 			lit = s.text[start..end]
 		}
+	}
 	return lit
 }
 
+fn trim_slash_line_break(s string) string {
+	mut start := 0
+	mut ret_str := s
+	for {
+		idx := ret_str.index_after('\\\n', start)
+		if idx != -1 {
+			ret_str = ret_str[..idx] + ret_str[idx+2..].trim_left(' \n\t\v\f\r')
+			start = idx
+		} else {
+			break
+		}
+	}
+	return ret_str
+}
+
 fn (s mut Scanner) ident_char() string {
 	start := s.pos
 	slash := `\\`