add fields for social usage on Lexeme

[dwhieb]

Add the following fields to the Lexeme, Form, and Sense.

• dialects
• locations

Update the documentation for the Lexeme's usages field to suggest that users utilize dialects and locations over usages whenever possible, reserving usages for things like register and social group.

Also update the Language's dialects field to mention that implementations may choose to validate the dialects fields on the Lexeme, Form, and Sense against its own values.