builtin: improve multi-platform portability for string.split_into_lines() (#17078)

vlib/builtin/string_test.v

@@ -883,7 +883,16 @@ fn filter(b u8) bool {
 }
 
 fn test_split_into_lines() {
-	line_content := 'Line'
+	line_content := 'line content'
+
+	text_cr := '${line_content}\r${line_content}\r${line_content}'
+	lines_cr := text_cr.split_into_lines()
+
+	assert lines_cr.len == 3
+	for line in lines_cr {
+		assert line == line_content
+	}
+
 	text_crlf := '${line_content}\r\n${line_content}\r\n${line_content}'
 	lines_crlf := text_crlf.split_into_lines()
 
@@ -899,6 +908,22 @@ fn test_split_into_lines() {
 	for line in lines_lf {
 		assert line == line_content
 	}
+
+	text_mixed := '${line_content}\n${line_content}\r${line_content}'
+	lines_mixed := text_mixed.split_into_lines()
+
+	assert lines_mixed.len == 3
+	for line in lines_mixed {
+		assert line == line_content
+	}
+
+	text_mixed_trailers := '${line_content}\n${line_content}\r${line_content}\r\r\r\n\n\n\r\r'
+	lines_mixed_trailers := text_mixed_trailers.split_into_lines()
+
+	assert lines_mixed_trailers.len == 9
+	for line in lines_mixed_trailers {
+		assert (line == line_content) || (line == '')
+	}
 }
 
 fn test_string_literal_with_backslash() {