Rename extension crate to extension_host (#20081)

This PR renames the `extension` crate to `extension_host`.

This is to free up the name so that we can create a smaller-scoped
`extension` crate.

Release Notes:

- N/A

crates/extension_host/build.rs

@@ -0,0 +1,46 @@
+use std::env;
+use std::fs;
+use std::path::PathBuf;
+
+fn main() -> Result<(), Box<dyn std::error::Error>> {
+    copy_extension_api_rust_files()
+}
+
+/// rust-analyzer doesn't support include! for files from outside the crate.
+/// Copy them to the OUT_DIR, so we can include them from there, which is supported.
+fn copy_extension_api_rust_files() -> Result<(), Box<dyn std::error::Error>> {
+    let out_dir = env::var("OUT_DIR")?;
+    let input_dir = PathBuf::from("../extension_api/wit");
+    let output_dir = PathBuf::from(out_dir);
+
+    println!("cargo:rerun-if-changed={}", input_dir.display());
+
+    for entry in fs::read_dir(&input_dir)? {
+        let entry = entry?;
+        let path = entry.path();
+        if path.is_dir() {
+            println!("cargo:rerun-if-changed={}", path.display());
+
+            for subentry in fs::read_dir(&path)? {
+                let subentry = subentry?;
+                let subpath = subentry.path();
+                if subpath.extension() == Some(std::ffi::OsStr::new("rs")) {
+                    let relative_path = subpath.strip_prefix(&input_dir)?;
+                    let destination = output_dir.join(relative_path);
+
+                    fs::create_dir_all(destination.parent().unwrap())?;
+                    fs::copy(&subpath, &destination)?;
+                }
+            }
+        } else if path.extension() == Some(std::ffi::OsStr::new("rs")) {
+            let relative_path = path.strip_prefix(&input_dir)?;
+            let destination = output_dir.join(relative_path);
+
+            fs::create_dir_all(destination.parent().unwrap())?;
+            fs::copy(&path, &destination)?;
+            println!("cargo:rerun-if-changed={}", path.display());
+        }
+    }
+
+    Ok(())
+}