Adopt vcpkg and streamline the build process

[yvt]

This PR makes the following changes to OpenSpades' standard build process for macOS and Windows.

• It now uses vcpkg to fetch and build external dependencies. This makes their builds fully reproducible and reviewable and obviates the need to trust the prebuilt library packages' maintainer.
  • Closes Consider using Vcpkg for managing the external dependencies on Windows and macOS builds #739.
  • vcpkg brings the latest versions of the libraries, which closes Japanese IME and Korean Input do not work on Windows #802.
• The CMake build process for macOS now produces a complete application bundle, which is almost ready for distribution.
• The macOS build process now automatically downloads required assets.
• The external dependencies are now built as static libraries. This means the following benefits:
  • Their .dll files no longer clutter the Windows binary distribution.
  • Static linking decreases the total code size.
• The Xcode project has been removed because the CMake build process can do the job.
• The new CI pipeline runs in GitHub Actions.

Tasks

• Get the Windows build working
• Get the macOS build working
• CI: Windows builds
• CI: macOS builds
• Document the revised build process for Windows
• Document the revised build process for macOS
• Get rid of the Externals directory
• Update the publishing process

[yvt]

Static linking in action (left: the latest release, right: this PR)