When the desktop support flag is enabled we should add checks for the current host to flutter doctor.

Currently since the build script is completely unopinionated (just running a shell script) we don't know all the things we'll want to add yet, but there are some things we could start with that we can be pretty confident will be necessary:

• On macOS, ensure that Xcode is installed, including command line build support.
• On Windows, ensure that VS code is installed and vcvars64.bat is locatable (for doing command-line builds)

For Linux we'll eventually want to check for the compiler and relevant libraries, but that will require decisions about what toolchain we're going to use for the created project on Linux. For now, we can just add the scaffolding for Linux without any checks.

CC @jonahwilliams