std.fmt: Add {q} format specifier

`q` meaning "quoted", handy for printf-debugging; especially where `\r`s are involved.
Generally useful when writing string manipulation code.
```zig
var s: []const u8 = "hello\x00world\r\n";
std.debug.warn("Content: {q}\n", .{s});
// Content: "hello\x00world\r\n"
```