Update to RGBDS 1.0 syntax and features

The tutorial has been written gradually over years by different authors, and in that time, RGBDS syntax has been through pretty many changes. Now that it's reached v1.0.0 the syntax is stable enough that I think it would be worth going through the code in the tutorial and ensuring that it's valid and idiomatic for the latest RGBDS.

(For example, [the Galactic Armada Makefile](https://github.com/gbdev/gb-asm-tutorial/blob/master/galactic-armada/Makefile) has `$(GFX) -c "#FFFFFF,#cfcfcf,#686868,#000000;"`, which would now idiomatically be `$(GFX) -c dmg`, and which could be enabling `-Weverything` warning diagnostics. The code itself might all still be simple enough -- no fancy macros or metaprogramming -- that nothing would need change, but the CLI usage at least could be improved.)

See also #125