Add context menu contribution point to Watch Variables Pane

[theIDinside]

The title is pretty much self-explanatory. It would essentially work identically to how the "normal" variables list work, since we have variableReferences, expressions and all that stored in the "containers".

This would be a far superior solution than adding individual items as suggested by this issue #133284

That way debug adapters can add custom actions like "format this value like so" or "this watch variable has this behavior" by adding a context menu action that gets the clicked item's data passed to it.