Added support for extern declarations and enhanced .lyng.d merging

- Implemented `extern` support for functions, classes, objects, enums, and properties in the `MiniAST`.
- Updated `MiniAST` to include `isExtern` field for applicable nodes.
- Enabled merging of `.lyng.d` declaration files into main `.lyng` scripts.
- Adjusted tests to validate `extern` behavior and documentation handling.
- Minor fixes to parser logic for improved robustness.

lyng-idea/src/main/resources/META-INF/plugin.xml

@@ -42,7 +42,7 @@
 
     <extensions defaultExtensionNs="com.intellij">
         <!-- Language and file type -->
-        <fileType implementationClass="net.sergeych.lyng.idea.LyngFileType" name="Lyng" extensions="lyng" fieldName="INSTANCE" language="Lyng"/>
+        <fileType implementationClass="net.sergeych.lyng.idea.LyngFileType" name="Lyng" extensions="lyng;lyng.d" fieldName="INSTANCE" language="Lyng"/>
 
         <!-- Minimal parser/PSI to fully wire editor services for the language -->
         <lang.parserDefinition language="Lyng" implementationClass="net.sergeych.lyng.idea.psi.LyngParserDefinition"/>