Run 'cargo package' in CI

Should verify the .crate file sent to crates.io can at least be built, which should catch things like the docs.rs build failure, as discussed in imgui-rs#645 and imgui-rs#631

.github/workflows/ci.yml

@@ -151,6 +151,16 @@ jobs:
       - name: build documentation
         run: cargo doc
 
+      # Test the .crate sent to crates.io can be built
+      - name: build .crate package (default)
+        run: cargo package
+      - name: build .crate package (docking)
+        run: cargo package --features docking
+      - name: build .crate package (freetype)
+        run: cargo package --features freetype
+      - name: build .crate package (all)
+        run: cargo package --features freetype,docking
+
       # Run unreasonably slow tests under release, but only the crates that have
       # them, and don't bother doing this on most platforms.
       - run: cargo test -p imgui --release -- --ignored