devex: add '{repo}' variable for extension paths

Previously '{source}' was added but this is a bit difficult to work
with since it's the path from the source file, which is usually the
'src/' directory under the repository. This won't be obvious anyone
who doesn't already know this. '{repo}' just points to the path of
the repository itself.

src/backend/src/boot/RuntimeEnvironment.js

@@ -222,6 +222,7 @@ class RuntimeEnvironment extends AdvancedBase {
         const environment = {};
         environment.source = this.modules.path_.dirname(
             this.entry_path || require.main.filename);
+        environment.repo = this.modules.path_.dirname(environment.source);
 
         const config_path_entry = this.get_first_suitable_path_(
             { pathFor: 'configuration' },