extension_host: Replace backslashes with forward slashes for cwd on Windows (#38072)

Instead of passing CWD verbatim from the Windows host with backslashes
and all, we now rewrite it into a more POSIX-happy format featuring
forward slashes which means `std::path::Path` operations now work within
WASI with Windows-style paths.

Release Notes:

- N/A

extensions/test-extension/src/test_extension.rs

@@ -18,6 +18,10 @@ impl TestExtension {
 
         let current_dir = std::env::current_dir().unwrap();
         println!("current_dir: {}", current_dir.display());
+        assert_eq!(
+            current_dir.file_name().unwrap().to_str().unwrap(),
+            "test-extension"
+        );
 
         fs::create_dir_all(current_dir.join("dir-created-with-abs-path")).unwrap();
         fs::create_dir_all("./dir-created-with-rel-path").unwrap();