Add a setting that exports all variables by default, regardless of
whether they use the `export` keyword. This includes assignments as well
as parameters.
Just does dependency analysis of variable uses, allowing variables to be
used out of order in assignments, as long as there are no circular
dependencies.
However, use of environment variable is not known to Just, so exported
variables are only exported to child scopes, to avoid ordering dependencies,
since dependency analysis cannot be done.
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.
