Enable no-angle-bracket-type-assertion

[ThomasdenH]

Since angle bracket type assertions were replaced by the as syntax, it's probably a good idea to use this instead. See also the rationale on the lint page.

[ghost]

Searching = <\w+> on DefinitelyTyped shows 107 results -- fixing manually would be doable since a lot of those are copy-pasted. Not sure if this is worth it though -- @RyanCavanaugh thoughts?

[ThomasdenH]

There are other ways of casting, like in (<HTMLElement>element).accessKeyLabel. It's probably quite hard to write an all-encompassing RegEx...

[ThomasdenH]

Adding it as a warning at first is also a possibility, this would be less intrusive and give time to update current definitions.

[RyanCavanaugh]

I'd be OK with this PR if it just turned it on for packages in which it wouldn't create a new warning (IOW make this change + disable it in folders where it needs to be disabled)