Support explicitly setting pointer tag to zero

[t4t5]

I noticed that if you try creating an inscription where you explicitly want the pointer to be 0, like this:

const inscription: Inscription = {
  tags: {
    contentType,
    pointer: 0n,
  },
  body,
};

p2tr_ord_reveal(u8RevealPublicKey, [inscription]);

...the library will simply ignore the pointer tag and not encode it (presumably because it's assumed to be the default?)

However, being able to explicitly define a pointer of 0 is very useful in cases where you want to inscribe something on a rare sat, where the first input might not contain the inscription witness.

Below is an example transaction. If pointer: 0 wasn't set, the inscription would be made on the first sat of the remaining fee output, instead of on the rare sat:
https://ordiscan.com/tx/bd63b470242919cbd32de382ba91d2f5140d5296dc2d0ff2c7c8a3deaaa387c1

I made a quick fix for this and wrote a simple test for it, but there might be a more elegant way of solving this.