Overlay 97 progress (like 2/3 of it)

[nathanfallet]

Decompiling like 2/3 of overlay 97. I remembered I started that but I hadn't time to finish it. I'll rework what is already done if code can be improved, and I'll come back later to finish the functions I haven't done yet (but I'm in a busy period with work so I prefer PR what I've already done than waiting for having the whole overlay done)

PR checklist

• This comment contains a description of changes (with reason).
• The ROM compiles to matching locally (make compare_heartgold && make compare_soulsilver).
• All C code is correctly formatted (git clang-format).
• This pull request is labeled according to the kind of work it represents.
• New or updated code labels follow the style guide
• This work adheres to the AI policy.
• The author has joined the pret discord server and sent a message in #pokeheartgold to verify that they are human