The `code` directive highlights all sources as GDScript. Other languages are
highlighted incorrectly at the moment, even if `.. code:: [language]` is
specified.
It does, however, work with the `code-block` directive. The reason seems to be
that this directive is Sphinx specific.
4-space is our convention for indented blocks and should be used consistently.
The only exception is for C++ code which is tab-indented, as the engine code.
