Support custom token/lexeme attribute for vectors

[adrianeboyd]

Description

Support custom token/lexeme attribute for vectors.

The main goal is to support LOWER or NORM for vector lookups instead of only ORTH. Because this attribute should be determined by the vectors themselves (primarily how the data was normalized for training) rather than the model config, the attribute for model vector lookup is moved from StaticVectors to a property of the Vectors.

Because a Token can have a custom NORM that overrides the Lexeme NORM, Token.get_struct_attr is used to retrieve the attr value in contexts where a Token is available.

Types of change

Enhancement.

Checklist

• I confirm that I have the right to submit this contribution under the project's MIT license.
• I ran the tests, and all new and existing tests passed.
• My changes don't require a change to the documentation, or if they do, I've added all required information.

[honnibal]

All makes sense and looks good 👍