Djblets.ExtensionHook has two issues that make it difficult to build

and use hook implementations.
The first is that defaults was a hard-coded ExtensionHookAttrs,

rather than a Partial<ExtensionHookAttrs>, which means that any

subclass must also include everything in ExtensionHookAttrs in their

defaults. This isn't necessary with Spina, which will merge the

dictionaries together. By switching to Partial<...>, subclasses can

type their own using their own attributes and rely on parent class

defaults.
The second issue is that ExtensionHook.each(), meant to iterate

through all instances of a hook for a given hook point, typed the hook

being passed to the callback as ExtensionHook and not as the subclass.

This forced callers to re-type, which was annoying. Now, we use generics

to reflect the specific subclass.

Unit tests pass.
Tested this with in-progress code in Review Board, verifying that the

typing was correct.