fix(prefixIds): dynamic functions should produce same prefix for given id

[SethFalco]

In Prefix IDs, we accept a function for the prefix parameter. However, the function is called every time an identifier is encountered, so instances of the same identifier are assigned different prefixes, breaking documents.

There are two solutions for this:

1. The plugin interface provides the identifier, which the user can use as a seed to generate a reproducible prefix for a given identifier.
2. We maintain an internal dictionary of prefixes, and refer to the cache when we encounter an identifier again.

I've opted for option 2 for now. Motivation:

• So the plugin works the way it was intended to be used already, without asking users to learn or change their existing configuration.
• Fits the more common use case. Most instances where users want prefixIds is to avoid ID collisions across multiple documents. Globally reproducible prefixes would undermine that.
• Doesn't add any complexity when configuring the plugin. I hope to avoid potential bug reports around use/misuse, like if the prefix param should return the whole new identifier or only the prefix.

Related

• Closes Re-add prefix parameter on cleanupIds to fix unique ids #1763
• Closes Unique id is broken since v3 #1746