Add the initial CLAUDE.md file

[xermicus]

So others don't have to regenerate it again.

[koute]

Not against adding this, but it should be manually edited before committing so that it makes sense. For example:

• RV32EM is not what we target anymore (although 32-bit is still technically supported).
• Saying which exact Rust versions are used is useless, and will go out of date eventually.
• Incorrect or irrelevant points should be removed (e.g. 'linker.rs: Runtime linking of host functions' is incorrect and the naming scheme is mostly a historical artifact since I was originally basing this on wasmtime's API, or "Performs register allocation via regalloc2" is a minor detail and gross misrepresentation that will most likely confuse the AI, since register allocation is only done in a very specific case)
• The environment variable section is probably useless

etc.

If you're not sure about something feel free to just remove it. :P

[xermicus]

I thought claude may have some logic in what it puts inside this file when generating it. It did actually not too bad in creating the file for paritytech/revive. Apparently here it's just mainly AI slop.

Manually reviewed it and changed a few things. I don't know yet what exactly to put in there and what not. But the file is intended to be changed over time anyways.