Add minimal CI workflow using GitHub Actions

[rhysd]

Hi, thank you for maintaining this great crate.

This PR adds a minimal CI workflow which will help you notice build breaks easily. Since this crate is very popular (15720778 times downloaded recently), I think it is important to keep all tests pass successfully.

The README.md file contains a Travis CI badge so I guess this project used it. However Travis CI no longer works at this time.

This minimal GitHub Actions CI workflow does the following:

• Unit test
  • Runs cargo test with stable Rust on Linux, macOS, and Windows
• Formatting
  • Check code formatting with rustfmt with nightly toolchain since the configuration in rustfmt.toml is only available on nightly

Here is an example of workflow run on my fork: https://github.com/rhysd/encoding_rs/actions/runs/8777306722