style_keyword_datatype erroneously applied to default_nettype directives

[Wren6991]

This file:

`default_nettype none

module foo;
endmodule

`default_nettype wire

Reports this error:

6   | `default_nettype wire
    |                  ^^^^ hint  : keyword should be followed by a single space
    |                       reason: consistent style enhances readability

This seems to be a mistake as this is not a declaration.

It's common to end a file with a default_nettype wire for compatibility with vendor RTL which relies on implicit declarations.