Shards: Support installing shards on Windows

[oprypin]

• [x] [Use git's checkout feature directly to write out repo files](https://github.com/crystal-lang/shards/pull/435)
• [x] [Don't use POSIX-specific shell constructs](https://github.com/crystal-lang/shards/pull/436)
• [x] [Use Process.quote instead of the old platform-specific helper](https://github.com/crystal-lang/shards/pull/437)
• [x] [Don't use compile-time shell commands to determine build timestamp](https://github.com/crystal-lang/shards/pull/438)
• [x] [Windows: pass a flag to allow creating symlinks without admin rights ](https://github.com/crystal-lang/crystal/pull/9767)

[Sija]

This ain't closed, yet.

[oprypin]

And, for a big finale..!

• Add continuous testing (including Windows) using GitHub Actions
• Expand Windows support + fix all specs
  
  
  
  • (offshoot) Make rm_r able to delete read-only files on Windows
  
  

(no need to reopen this, shards installation technically already works)