Add CI, update example, dependencies, readme

[bookshiyi]

• Add CI

  • Run build test on Windows Linux macOS
  • Upload build target to artifact(GitHub Actions)

• README:

  • Add badges (GitHub and Pub)
  • Optimize markdown syntax
  • Update code block readability

• example:

  • Delete example web target
  • Change example target name to bitsdojo_window_example
  • Update example dependencies version

• dependencies version:

  • flutter_lints from ^1.0.0 to ^2.0.0
  • environment sdk from ">=2.17.0 <3.0.0" to ">=2.17.0 <4.0.0"