Gracefully handle missing irb/notifier

[lsegal]

Fixes #1649
Fixes #1636
Fixes #1645

[noraj]

This PR does not bring 100% of Ruby 4.0 support as seen in the v0.9.39 release note.

Add support for Ruby 4.0 (#1663)

Indeed, if using markdown formatting with commonmarker, it's still not possible to use yard and ruby 4.0. Because #1528 is not fixed, yard still only support commonmarker 0.9.x and not commonmarker 1.x or commonmarker 2.x. But because commonmarker 0.9.x does not support ruby 4.0 then yard does not support ruby 4.0. For 100% ruby 4.0 support, yard should support commonmarker 1.x and 2.x and close #1528. Is has been nearly two years that #1540 have not received any update.

[lsegal]

Feedback was already provided on #1540 that was never addressed. That PR should probably be closed as stale. It would be easy to open a PR that addresses the feedback by creating a proper compatibility layer and does not break backward compatibility. You're free to open such a PR.