Add a wasmtime::component::bindgen! macro (#5317)

* Import Wasmtime support from the `wit-bindgen` repo

This commit imports the `wit-bindgen-gen-host-wasmtime-rust` crate from
the `wit-bindgen` repository into the upstream Wasmtime repository. I've
chosen to not import the full history here since the crate is relatively
small and doesn't have a ton of complexity. While the history of the
crate is quite long the current iteration of the crate's history is
relatively short so there's not a ton of import there anyway. The
thinking is that this can now continue to evolve in-tree.

* Refactor `wasmtime-component-macro` a bit

Make room for a `wit_bindgen` macro to slot in.

* Add initial support for a `bindgen` macro

* Add tests for `wasmtime::component::bindgen!`

* Improve error forgetting `async` feature

* Add end-to-end tests for bindgen

* Add an audit of `unicase`

* Add a license to the test-helpers crate

* Add vet entry for `pulldown-cmark`

* Update publish script with new crate

* Try to fix publish script

* Update audits

* Update lock file

Cargo.toml

@@ -133,6 +133,7 @@ wasi-tokio = { path = "crates/wasi-common/tokio", version = "=5.0.0" }
 wasi-cap-std-sync = { path = "crates/wasi-common/cap-std-sync", version = "=5.0.0" }
 wasmtime-fuzzing = { path = "crates/fuzzing" }
 wasmtime-jit-icache-coherence = { path = "crates/jit-icache-coherence", version = "=5.0.0" }
+wasmtime-wit-bindgen = { path = "crates/wit-bindgen", version = "=5.0.0" }
 
 cranelift-wasm = { path = "cranelift/wasm", version = "0.92.0" }
 cranelift-codegen = { path = "cranelift/codegen", version = "0.92.0" }
@@ -162,6 +163,7 @@ wasmprinter = "0.2.44"
 wasm-encoder = "0.20.0"
 wasm-smith = "0.11.9"
 wasm-mutate = "0.2.12"
+wit-parser = "0.3"
 windows-sys = "0.42.0"
 env_logger = "0.9"
 rustix = "0.36.0"
@@ -179,6 +181,7 @@ tracing = "0.1.26"
 bitflags = "1.2"
 thiserror = "1.0.15"
 async-trait = "0.1.42"
+heck = "0.4"
 
 [features]
 default = [