Fix key reuse bug

[Hopding]

What?

Fixes a bug introduced in #1033 that allows Font, XObject, etc... keys to be overwritten.

Why?

Fixes #1075.

How?

Continue to use deterministic random number generation, but check if the generated key conflicts with an existing one. Keep generating new keys until there's not a conflict.

Testing?

Added unit tests and ran integration tests.

New Dependencies?

No

Screenshots

N/A

Suggested Reading?

Yes

Anything Else?

N/A

Checklist

• I read CONTRIBUTING.md.
• I read MAINTAINERSHIP.md#pull-requests.
• I added/updated unit tests for my changes.
• I added/updated integration tests for my changes.
• I ran the integration tests.
• I tested my changes in Node, Deno, and the browser.
• I viewed documents produced with my changes in Adobe Acrobat, Foxit Reader, Firefox, and Chrome.
• I added/updated doc comments for any new/modified public APIs.
• My changes work for both new and existing PDF files.
• I ran the linter on my changes.