Document obsolete removed asm syntax features and CLI flags

[Rangi42]

For years we've been gradually deprecating and removing redundant, limited, or just plain buggy features of the RGBASM language, while introducing more extensible alternatives. This is great for the eventual 1.0.0 release, but it does mean that older users keep going through stages of updates, and old projects may end up having a lot of backwards-incompatible syntax.

It would be helpful to document the various things we've removed (DEF-less constants/variables, automatic nop after halt, PRINTV, rgbgfx -h, and so on) in a single place, rather than a bunch of release notes. I went ahead and listed them: https://pastebin.com/0aifLEJD Categorized as Removed, Replaced, Changed, and Never Supported (that last one is just for ldhl, which I kept thinking we once supported but never in fact did).

I'm not sure where to publish this. It would be nice to have on the rgbds.gbdev.io website. The easiest way to do that would be to reformat as a man page. The content is arguably relevant to the History page (rgbds.7); otherwise it would need a new one (rgbasm.7? rgbz80.7?).