fix(cleanupIds): skip generated ids for malformed references

[SethFalco]

We had an issue where if there were nodes with malformed references that pointed to a node that didn't exist, this would unintentionally duplicate an object.

For example, if a document had <use href="#a"/>, but there was no node defined with the id of a. The plugin would've assumed a is an available ID and assigned this to a node and all references, but this previously malformed reference would end up becoming valid and pointing to the node that was just assigned an id of a.

In other words, nodes that previously had a bad reference, now reference a node and duplicate whatever the definition was.

This PR checks if, for a generated ID, is there an existing reference that doesn't point to a node. When true, back off and generate the next ID.

Chores

I also had some chores pending which were trivial individually, I opted to incorporate them as part of this PR in separate commits.

• In JSDocs, use the nullable syntax as it's more concise and familiar from TypeScript usage.
• discord → Discord (proper nouns should match capitalization)
• Use actions/checkout v4 instead of v2

Related

• Closes cleanupIDs uses id twize #1195