Add CI check for missing listener docs

[cocosolos]

I affirm:

• [x] I understand that if I do not agree to the following points by completing the checkboxes my PR will be ignored.
• [x] I understand I should leave resolving conversations to the LandSandBoat team so that reviewers won't miss what was said.
• [x] I have read and understood the Contributing Guide and the Code of Conduct.
• [x] I have tested my code and the things my code has changed since the last commit in the PR and will test after any later commits.

What does this pull request do?

Adds a job to scan for undocumented listeners so stuff like #5961 is unnecessary.

Steps to test these changes

$ bash tools/ci/cpp.sh src/map/ai/controllers/mob_controller.cpp 2>> cpp_checks.txt
src/map/ai/controllers/mob_controller.cpp:1002: Found undocumented listener. Please document this in AI_Events.txt.
                    PMob->PAI->EventHandler.triggerListener("ROAM_ACTION", CLuaBaseEntity(PMob));

src/map/ai/controllers/mob_controller.cpp:1100: Found undocumented listener. Please document this in AI_Events.txt.
            PMob->PAI->EventHandler.triggerListener("PATH", CLuaBaseEntity(PMob));

src/map/ai/controllers/mob_controller.cpp:1134: Found undocumented listener. Please document this in AI_Events.txt.
        PMob->PAI->EventHandler.triggerListener("WEAPONSKILL_BEFORE_USE", PMob, wsid);

[cocosolos]

This doesn't cover listeners defined and used exclusively in Lua (AERN_RERAISE, JOB_SPECIAL_). I don't think it's necessary to document those as they're defined and used for specific cases.