Blob: Add types

[sirreal]

Description

Part of #18838
Extracted from #18942

Add types to blob package.

Testing Instructions:

Verify type checking passes:

npm run lint-types

[sirreal]

Initially, my editor ignored the | undefined part of this type and the following one. Adding explicit strictNullChecks to the tsconfig restored it.

[sirreal]

{?File} is a nullable type in JSDoc, equivalent to File | null, which isn't accurate.

[aduth]

{?File} is a nullable type in JSDoc, equivalent to File | null, which isn't accurate.

Yeah, I think you'll find a lot of examples like this in the codebase, where nullable type is used incorrectly as interchangeable with "optional" or "possibly undefined".

[sirreal]

This should be enabled via strict, but I some Type | undefined types didn't seem to be handled correctly. Adding this seems to fix the issue.

I wonder if strict + noImplicitAny results in odd behavior.