Rework Card::counters to prevent direct edit access

Followup of #12314 (similar work, for the Player's counters)

Direct access to the Counters to add/remove counters is ill advised, as it avoid any kind of prevention/event firing.

Of note, the current implementation of [[Tawnos's Coffin]] purposely doesn't fire COUNTER_ADDED event, and it is not clear to me that's correct at this time.

Start with Card::counters and compare the related work in #12314, then inspect the current handling of direct counter additions and removals. The issue is complete when those operations no longer bypass prevention or events, with the Tawnos's Coffin COUNTER_ADDED behavior explicitly resolved.

Written by the indexing model from the issue text.