Support installing shards on Windows

[oprypin]

• [x] Use git's checkout feature directly to write out repo files
• [x] Don't use POSIX-specific shell constructs
• [x] Use Process.quote instead of the old platform-specific helper
• [x] Don't use compile-time shell commands to determine build timestamp
• [x] Windows: pass a flag to allow creating symlinks without admin rights

[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)