shorebird cli should support windows

[felangel]

Currently the shorebird tooling only works on MacOS (because of the shorebird engine).

Todo

• build different versions of the shorebird engine for each platform/arch
• pull the correct engine based on the host platform
• proxy to Flutter with the correct args/paths

[eseidel]

This is blocked by #56 as it will be really hard to maintain multiple platforms w/o some form of automation.

[eseidel]

We believe linux now works, but need to test. Windows is missing an installer. Interesting to note that Rust doesn't use a curl | sh method on windows (perhaps there is no equivalent):
https://forge.rust-lang.org/infra/other-installation-methods.html

[eseidel]

The big thing missing from Windows is a shorebird.bat (or shorebird.ps1?) like Flutter's:
https://github.com/flutter/flutter/blob/master/bin/flutter.bat

[eseidel]

Filed shorebirdtech/install#10 and shorebirdtech/install#11

[eseidel]

Looking through shorebird_cli we have runInShell sprinkled in various places. I suspect we need to centralize that handling (in part to deal with Windows).

[bryanoltman]

#369 makes an already-installed shorebird work in powershell. I think the only remaining task is the installer.