chore: rename rule to ad-hoc-sub-process

[nikku]

Proposed Changes

As a follow-up to #189, rename the new rule to ad-hoc-sub-process. See prior art. We refer to sub-process predominantly, and -activity does not add any more value.

Checklist

To ensure you provided everything we need to look at your PR:

• Brief textual description of the changes present
• Visual demo attached
• Steps to try out present, i.e. using the @bpmn-io/sr tool
• Related issue linked via Closes {LINK_TO_ISSUE} or Related to {LINK_TO_ISSUE}

[jarekdanielak]

For some reason I thought there would be some conflict between this and the rule from bpmnlint, but the rules are always prefixed with the plugin name. Thanks for the adjustment.