fix: export ArrayChange type

[bdkopen]

Adds ArrayChange to the type exports. This was originally exported as part of the @types/diff package (code), but isn't exported here. Including it will make the v8 type update simpler.

Alternatively, we could cut the type altogether since it hasn't been exported since the v8 release.

[ExplodingCabbage]

Huh, interesting. It's a bit odd that I included a type with the same name here but make the type slightly different (not parameterized like the old DefinitelyTyped version) and didn't export it; clearly what I did doesn't make sense and must've been a mistake.

I think I see three options here would seem to be:

• export it, as this PR does
• just remove it, as you suggest - right now it's neither exported nor used internally
• export it and make it parameterized (ArrayChange<T> instead of just ArrayChange) like the old version

[ExplodingCabbage]

I think I'll go for the third option, just to minimise the compatibility break. Versions 5 and 7 still collectively have 15x the downloads that version 8 has, so making the upgrade path more painless does still have value, notwithstanding that it's been months since I released version 8.