feat: Windows support in makefiles

[SecondFlight]

Related to #33. Supersedes #35, since workflows aren't running on that PR.

This patch adds Windows support for the Makefiles, while trying to preserve existing behavior on macOS and Linux. There are likely more things wrong, but it's working until the first Windows-related error in rid itself.

Notable changes:

• There is now a Makefile.variable file, which is imported by the other Makefiles. This allows variables to be defined once and used anywhere that imports Makefile.variable.
• If pub is missing, the script now checks for flutter pub before failing.

[SecondFlight]

Well, workflows aren't running here either - my bad. Reopening #35...