feat(bloc_lint): avoid_duplicate_event_handlers

[felangel]

Description

As a developer, I want to be warned if I have duplicate event handlers registered for a given bloc (originally referenced in #4264)

[vsc9729]

Hey Felix. I want to work on this issue as a first issue. I've worked on a couple flutter projects before and I want this to be my first issue. Can I take up this issue?

[felangel]

Hey Felix. I want to work on this issue as a first issue. I've worked on a couple flutter projects before and I want this to be my first issue. Can I take up this issue?

Sure I’ve assigned you, thanks!