Add tres_null_meaning

[SuperAuguste]

pub const A = struct {
    pub const tres_null_meaning = .{.{"a", .field}, .{"b", .null}, .{"c", .dual}};
    a: ?u8 = null,
    b: ?u8 = null,
    c: ??u8 = @as(?u8, null),
};

becomes

{"b": null, "c": null}

Useful for differentiating between abc? fields and abc: ... | null LSP fields.

?? dual fields are non-existent on the first null, then null on the second null. This eliminates the need for Undefinedable which is just super clunky to use.