Make it easier to activate/deactivate Antora support

[ggrossetie]

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• Replace the current user/workspace setting by "Show a prompt to enable Antora support when an antora.yml file is detected". yes/no
• Add a command to toggle Antora support on a given workspace (saved in Memento)

That way we can control if you want a prompt when the extension detects an antora.yml file. We will also have the ability to enable/disable Antora support using a command (in case we clicked "no" on the prompt or in case the extension didn't detect the antora.yml or in case we want to turn it off)