Reorganize the ASM/src directory

[mzxrules]

The goal for this PR is to sort .asm files into two folders (base and payload) with the hopes that it will someday make it easier to convert baserom modifications into C using Decomp. This PR avoids creating differences in rom_patch.txt and asm_symbols.txt so that in-game testing is not required. However, note that this PR includes the fix from #2352.

Once merged, the next step is to do a more thorough clean up code in the base game files, and break up hacks.asm.

For the base folder, the idea is to roughly match the directory layout of decomp:

• Modifications that edit the "code" segment should be split into separate files named after the source file within the code segment that they modify, due to the large number of source files within the segment.

• Otherwise, new base game .asm files should be named after the segment they modify.

For the payload folder, I'm not really sure how best to organize it. I mainly stuck a handful of seemingly important files into the root of payload, stuck drop_overrides in it, and put everything else into the mods folder.

[flagrama]

I recommend adding the desired way to use these folders to ASM/README.md.

[mzxrules]

I feel like that sort of information doesn't belong in README.md. There's talk of creating a CONTRIBUTING.md but I don't feel comfortable starting one.

[fenhl]

CONTRIBUTING.md exists now, feel free to add a section about this there.