Version dune-workspace and user config files

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The last files that are not versioned are dune-workspace files and the user configuration file in ~/.config/dune/config. It seems worth versioning them before the 1.0.0 otherwise it will be hard to do it afterwards. It is a bit more complicated to version those and keep backward compatibility, so I suggest the following plan:

• jbuilder ignores dune-workspace files
• jbuilder parses workspace files in jbuild syntax
• jbuilder accepts a user configuration file without (lang ...) and in this case parses it in jbuild syntax
• dune ignores jbuild-workspace and jbuild-workspaceXXX files
• dune requires (lang dune ...) in workspace files
• dune requires (lang dune ...) in the user configuration file

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This is now done