Add a sanity check for gcode: formatting error

I have noticed a common problem when helping others with their .cfg for various things. When adding g-code in a macro or other configs. there is no sanity check error for bad formatting if it is in this format;

runout_gcode: PAUSE

instead of;

runout_gcode:
    PAUSE

I know this is a read the docs issue, but with all the copy paste that goes on, an indent or missed new line can go unchecked. if it makes the code fall on the same line as ´´´gcode:´´´
I don’t know if this will get to the devs to check out but it would be a handy addition to warn the user that the gcode format is wrong instead of it just ignoring what is on the same line.