Warn but don't skip if Asciidoctor extension is registered as Antora extension (3.1.x only)

There are some Asciidoctor extensions that provide a single Asciidoctor and Antora entrypoint (i.e., a hybrid extension). One example is asciidoctor-treeview. Antora detects this as an Asciidoctor extension and refuses to register it. This is the new behavior we want, but adding it to 3.1.x is a breaking change and thus should be reverted. Antora 3.1.x should only warn about the situation and precede with registering it. This change should only be applied to the v3.1.x branch.

Asciidoctor extensions designed to be used with Antora should always provide a dedicated entrypoint for use with Asciidoctor and Antora (e.g., asciidoctor-acme and asciidoctor-acme/antora-extension). The Antora extension can automatically register the Asciidoctor extension so that the user only needs to configure a single entry point. But the entry point should not be a hybrid. Otherwise, Antora cannot detect whether it's a valid extension Antora extension. It's also a confusing situation for the user.

Related to this change, the terminology "AsciiDoc extension" in the log messages should be updated to "Asciidoctor extension" to match the documentation and configuration schema.