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

crates/wit-bindgen/Cargo.toml

@@ -0,0 +1,13 @@
+[package]
+name = "wasmtime-wit-bindgen"
+version.workspace = true
+authors.workspace = true
+description = "Internal `*.wit` support for the `wasmtime` crate's macros"
+license = "Apache-2.0 WITH LLVM-exception"
+repository = "https://github.com/bytecodealliance/wasmtime"
+documentation = "https://docs.rs/wasmtime-wit-bindgen/"
+edition.workspace = true
+
+[dependencies]
+heck = { workspace = true }
+wit-parser = { workspace = true }