Add a macro version of trixi_include to use the right module automatically

[vchuravy]

I just ran into the footgun that in tests or in Documenter trixi_include uses the wrong module.
Using a macro we can query the calling module with __module__ and allow for "prettier" usage.

I personally prefer macros for "magic" functionality like trixi_include

[JoshuaLampert]

Nice idea. Can you also export the macro, please?

[ranocha]

Thanks, I like the idea. Could you please

• export the macro to fix CI
• add a test for the error path in the macro
• add a test with multiple keyword arguments

? Programming macros is always a bit scary in Julia compared to normal functions, so I would like to have a bit better test coverage for it to make it easier for people working with the code in the future.