feature: case statements

[Alizter]

We add a (case ...) constructor to dune_lang for actions which when expanded will reduce down to the matching branch. Basic use looks like:

 (rule
  (alias foo)
  (action
   (case %{read-lines:version}
    (1 (echo "version is 1"))
    (2 (echo "version is 2"))
    (3 (echo "version is 3"))
    (_ (echo "shouldn't happen, got %{read-lines:version}")))))

which together with a version file containing 2 would print 2.

• changelog
• tests
• Should the default case be optional? I think no.
• Multicase branches?
• Also implement cond?
  • will be done in followup PR, currently blocked by test: expand %{deps} in (cat) #8196
• addreses part of Adding some conditionals in the language #924

[Alizter]

I'm gonna open another PR at a later date with cond.