Add example for pattern matching on options

[konsumlamm]

Follow-up to #17036. This PR adds an example for pattern matching on options. It uses code-block, since fusion isn't bundled anymore (see #16925). If there is a way to turn this into a runnableExamples, please let me know.

[timotheecour]

If there is a way to turn this into a runnableExamples, please let me know

• the following works, but not inside a runnableExamples because of this (IMO easily fixable) bug that removes do from runnableExamples: runnableExamples transforms do: => : and (expr) => expr, producing invalid code #13491

when true:
  {.experimental: "caseStmtMacros".}
  when (compiles do: import fusion/matching):
    case some(42)
    of Some(@a):
      assert a == 42
    of None():
      assert false
    Some(Some(None())) := some(some(none(int)))

• however, even after we fix runnableExamples transforms do: => : and (expr) => expr, producing invalid code #13491, IMO this pattern when (compiles do: import fusion/matching): is bad, as it violates scoping rules: when foo: bar shouldn't inject declarations in bar, refs when foo: bar puts symbols declared in foo in scope #12030 and Stange issues for "when (compiles do: import a):" #16287; the correct fix is whichModule refs whichModule (subsumes canImport / moduleExists) timotheecour/Nim#376; so I suggest leaving is as you did and waiting for whichModule, maybe add: ### pending https://github.com/timotheecour/Nim/issues/376 use whichModule`

[timotheecour]

LGTM after final nits