Add functionality to `TermSelector` to prevent editors from creating new terms

[nikkifurls]

Description

Currently, the TermSelector component allows new terms to be created, if they don't already exist. This is handled via the __experimentalValidateInput prop on the FormTokenField Gutenberg component. When an editor enters a term that doesn't already exist, the new term is created and added to the post automatically.

We should add a prop on TermSelector that will prevent editors from creating new terms.

We should also display a message to editors when they enter a term that doesn't exist.

Use Case

Sometimes, taxonomy terms are managed programmatically, and editors are only allowed to add/remove them from posts, but not allowed to create/edit/delete them. In this case, it would be helpful for the TermSelector to prevent editors from creating new terms that don't already exist.