Add `just_version` function to get the version of the `just` tool

[nyurik]

While #2290 (minimal required just version) is being discussed, we may also want a separate path to ensure that the current runner meets our needs. Let's add a new function just_version() that returns a semver of the current just tool - this way recipes could use semver_matches(just_version(), "...") to see if just requirements are met for the project. Moreover, this could be added as some "prereq" recipe that most other recepes depend on - which will either succeed or fail with instructions to update just.

[private]
validate_just:
    if [ {{semver_matches(just_version(), ">1.38")}} != 'true' ]; then \
        echo "Your just runner is too old, please update" ;\
        exit 1 ;\
    fi

foo: validate_just
    echo "running foo"

In the mean time, we could already use something like {{semver_matches(shell(just_executable(), '--version'), ">1.38")}} as a temporary workaround until a new function is added

[laniakea64]

Per #2892 (comment) :

The use case for a just_version() function is reporting or logging information about the execution environment for the purpose of making debugging easier. The just version needs to be made available internally (e.g. a just function or builtin constant) because on Windows and in some CI environments, the currently-available method of getting just version by doing {{quote(just_executable())}} --version does not reliably work.