format command for splitting long lines of .dsl file

Currently, the .dsl file generated by several tools use a single line presentation for all key-values

This is not really convenient for k3 key which lists all aspect classes it uses.

it could be improved by highlighting the fact that dsl file format supports the use of \ in order to break lines.
for example:
```
name = org.eclipse.gemoc.sample.legacyfsm.xsfsm.XSFSM
ecore = platform:/resource/org.eclipse.gemoc.sample.legacyfsm.xsfsm.xsfsm/model/XSFSM.ecore
k3 = org.eclipse.gemoc.sample.legacyfsm.xsfsm.xsfsm.aspects.StateAspect,org.eclipse.gemoc.sample.legacyfsm.xsfsm.xsfsm.aspects.StateMachineAspect,org.eclipse.gemoc.sample.legacyfsm.xsfsm.xsfsm.aspects.TransitionAspect
```

would become:
```
name = org.eclipse.gemoc.sample.legacyfsm.xsfsm.XSFSM
ecore = platform:/resource/org.eclipse.gemoc.sample.legacyfsm.xsfsm.xsfsm/model/XSFSM.ecore
k3 = org.eclipse.gemoc.sample.legacyfsm.xsfsm.xsfsm.aspects.StateAspect,\
org.eclipse.gemoc.sample.legacyfsm.xsfsm.xsfsm.aspects.StateMachineAspect,\
org.eclipse.gemoc.sample.legacyfsm.xsfsm.xsfsm.aspects.TransitionAspect
```

Ideally, the command should be mapped to the classic right click -> Source -> format popup menu (and Ctrl + Maj + Click)

Start by locating the .dsl editor and the existing Source context-menu actions in the repository, then trace how editor commands are registered. Done means a format command splits long values using the supported backslash continuation syntax and is available from the requested right-click Source menu and keyboard shortcut.

Written by the indexing model from the issue text.