If extension ID is not specified, default to value of require key

When an extension is declared in the playbook, if the id key is not specified, the value should default to the value of the require key. This way, it's not necessary to repeat the value when using this sensible default is sufficient.

Example:

antora:
  extensions:
  - require: '@antora/pdf-extension'
    enabled: false

If an extension with that ID already exists, then the ID should not be assigned automatically.

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