feat: add shared Rust MSIX engine and CLI

Closes #2

src/fixtures/desktop-app.rs

@@ -0,0 +1,11 @@
+#![windows_subsystem = "windows"]
+
+#[link(name = "user32")]
+extern "system" {
+    fn MessageBoxW(window: *mut std::ffi::c_void, text: *const u16, caption: *const u16, kind: u32) -> i32;
+}
+fn main() {
+    let title: Vec<u16> = option_env!("FIXTURE_NAME").unwrap_or("MSIX fixture").encode_utf16().chain(Some(0)).collect();
+    let text: Vec<u16> = "This is a packaging test application.".encode_utf16().chain(Some(0)).collect();
+    unsafe { MessageBoxW(std::ptr::null_mut(), text.as_ptr(), title.as_ptr(), 0); }
+}