move generated bindings files to a `generated/` sub-directory

This is a small pet-peeve of mine but I figured I'd raise it: I find the name `src/bindings.rs` to be a little ambiguous, as it assumes people are already familiar with how WIT, Wasm Components, and `wasm-bindgen` work. It also assumes that people seeing the file in the hierarchy understand that this was auto-generated and was not checked in by hand. I feel like we would be better served by clearer labeling that this file is automatically generated, and specific to Wasm Components - not just any old bindings. This is doubly the case when we consider that people might generate bindings to other things, for example: it's not unreasonable for someone to generate bindings to a protobuf schema, and then wire that over `wasi:sockets`.

What I'm proposing here are two relatively minor changes:

1. Create a directory called `src/generated/` which we recommend using as the output target for all generated code.
2. Rename the `bindings.rs` file to something slightly more descriptive. Something like `wasm_bindings` or `component_bindings` could more clearly clue in people unfamiliar with Wasm Components about what is going on.

If we put those two together, that would result in a rename like `src/bindings.rs` → `src/generated/component_bindings.rs`. A little longer, but more clearly factored in a way that makes it clear we're not supposed to edit this by hand.

This is clearly a "what color do we paint the bike shed" type of question, which is why I've put this off raising this for a while. But it feels like we're in the home stretch of delivering an end-to-end story, and I've recently seen at least one person be confused about the purpose of `bindings.rs`, so I figured raising this now might actually be reasonable timing.