support Windows clipboard on WSL

[ahihi]

adds support for interacting with the Windows clipboard when running Orca in WSL (Windows Subsystem for Linux).

caveats:

• unfortunately, clip.exe has no way to retrieve the clipboard content. here a powershell command is used instead, which is quite slow.
  • in practice however, at least in Microsoft's Terminal app, pressing ctrl+v invokes a built-in paste function which works fine. ctrl+alt+v will send ctrl+v to the running program, invoking the slow powershell paste in Orca.
• im not sure if my method of detecting WSL in the build script is ideal
• i suppose a user might possibly want to run Orca in WSL and still use xclip. this is currently not possible with my code