zon: add Formatter

[benburkert]

Add zon.Formatter & zon.fmt for pretty printing:

const Thing = struct {
    num: u32,
    msg: []const u8,
    sub: struct {
	mybool: bool,
    },
};

const thing: Thing = .{
    .num = 927,
    .msg = "hello",
    .sub = .{ .mybool = true },
};

std.debug.print("{f}", .{std.zon.fmt(thing, .{})});

// outputs:
//
// .{
//     .num = 927,
//     .msg = "hello",
//     .sub = .{ .mybool = true },
// }