Add conditional-vars extension and fix author name in focus-mode

[lsbjordao]

This PR adds the conditional-vars extension and fixes the author name in the focus-mode extension.

conditional-vars

Adds support for conditional content blocks based on variables defined in _variables.yml.

Unlike .when, which operates on profiles, this extension enables conditional rendering driven by project-level variables.

focus-mode

• Fixes author name metadata

[cderv]

Thank you

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