fix: add jcef as dependency for Intellij 2026.2

[heddxh]

Since 2026.2, jcef should be declared explicitly, or Intellij will complain java.lang.NoClassDefFoundError: com/intellij/ui/jcef/JBCefBrowser

See: asciidoctor/asciidoctor-intellij-plugin#2081

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[gemini-code-assist]

Code Review

This pull request adds a dependency on com.intellij.modules.jcef to the plugin.xml file. I have no feedback to provide as there are no review comments.