* Avoid $pkgname to make it easier to sync with mingw-w64-libchewing at
  https://gitlab.com/yan12125/aur/-/tree/master/mingw-w64-libchewing

* checkpkg indicates that pinyin.tab is removed, which is a result of
  https://github.com/chewing/libchewing/commit/d10f84fd1e8b96c304dc28256e63666cd0d51661

* Add Rust dependencies as upstream enables the Rust implementation by
  default in this version
* Refactor $pkgver to allow easier testing of release candidates
* Move the build directory to a subdirectory of the source directory.
  This is needed for Rust tests.

The minisign command is adapted from
https://gitlab.archlinux.org/archlinux/packaging/packages/libsodium/-/blob/main/PKGBUILD

See: https://github.com/chewing/chewing.github.com/issues/36

* List myself as the maintainer
* Mention chewing-editor in optdepends
* Add check()
* Style: tabs to spaces and remove unnecessary $srcdir

[1] https://github.com/chewing/libchewing/commit/c0414d2f1cc8869a811f49878808232b27d8bf5e