Fix issue 1428, Eclipse plugin commonmark dependency

[mihnita]

No description provided.

[mihnita]

Tested with the latest Eclipse (2026-09).

The current PR uses the org.commonmark included with Eclipse, and uses dependency-resolution/extraRequirements to tell the user that the org.commonmark plugin is required.
This seems cleaner, and we behave more like a standard Eclipse plugin.

The drawback is that we depend on the org.commonmark included with Eclipse,
whatever that is (0.30.0 for Eclipse 2026-09, 0.24.0 for Eclipse 2025-06, etc)

The alternative is to pack in the plugin the exact org.commonmark version we
tested the formatter with.
We would need the shaded jar for that, google-java-format-{version}-all-deps.jar.
Or "shade" the org.commonmark in the plugin itself.