Enable access to counters with identifier

[kunzaatko]

Currently, there is no clear way to update a counter manually.
An example of the desired usage:

#import "@preview/ctheorems:1.1.2": *
#show: thmrules

#let theorem = thmbox(
  "theorem",
  "Theorem",
  base_level: 0,
)

#theorem("Number 1")[This is a theorem]

#counter("theorem").update(6)

#counter("theorem").display()

#theorem("Number 7")[This is a theorem]

The outcome:

This should instead result with the second theorem be "Theorem 7". At the moment, I think the counters are only accessible via looking them up in the state and updating them manually with the knowledge of the data structure of the package. If there is no way around this, it should be at least documented, since it is a feature that is quite common in practice.