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/component-macro/tests/codegen/simple-functions.wit

@@ -0,0 +1,16 @@
+interface simple {
+  f1: func()
+  f2: func(a: u32)
+  f3: func(a: u32, b: u32)
+
+  f4: func() -> u32
+  f5: func() -> tuple<u32, u32>
+
+  f6: func(a: u32, b: u32, c: u32) -> tuple<u32, u32, u32>
+}
+
+world the-world {
+  import imports: simple
+  export exports: simple
+  default export simple
+}