Override MessageEventTarget's events' this-parameter type

[sandersn]

This adds the ability to override the this parameter of a type's event handlers, which I need for MessageEventTarget to avoid breaking code.
Technically, I believe this: this is correct for all event handlers, but it's expensive to compile and confusing to understand so I'd prefer to use it only when needed.

I don't know if this is the right way to do this in the generator code. Corrections welcome.

[github-actions]

Thanks for the PR!

This section of the codebase is owned by @saschanaz - if they write a comment saying "LGTM" then it will be merged.

[saschanaz]

Probably better to autodetect whether there's inheritance/mixin relationship and use this automatically in such case, wdyt? (In that case anything related to HTMLElement should be excluded for now or otherwise all the HTML event handlers would get this.)

[saschanaz]

But in case this is urgent I think this is fine too.