Add a `set SETTING := VALUE` construct.
This construct is intended to be extended as needed with new settings,
but for now we're starting with `set shell := [COMMAND, ARG1, ...]`,
which allows setting the shell to use for recipe and backtick execution
in a justfile.
One of the primary reasons for adding this feature is to have a better
story on windows, where users are forced to scrounge up an `sh` binary
if they want to use `just`. This should allow them to use cmd.exe or
powershell in their justfiles, making just optionally dependency-free.
