Add support for tscn files

This ticket is a proposal to handle tscn files differently than currently planned. see #174 (closed) .

proposed:

• file symbol for tscn files
• syntax highlighting for tscn files
• make tscn files runnable (this is actually more correct than .gd files, bc not every scene has a .gd file)
• many more options not listed yet (use export variables to show a config dialog etc.)