refactor: clearer custom timestamp generation logic

docs(readme): update generation behavior section with custom timestamp

refactor(encode): throw error in `encode` if id is invalid

refactor(encoding): remove `base-x`, manage encoding internally

feat(generation): support custom timestamp (#1)

* feat(generation): support passing custom timestamp

* test(generation): add custom timestamp test

* docs(readme): add custom timestamp sections

docs(readme): fix broken new issue link

docs: update references from draft to RFC 9562

refactor(generation): get `rand_a` part from `crypto.randomUUID()`

build(package): remove `packageManager` property

refactor(generation): update max `rand_b` random increment value to 2^32