Dialects: add basison (BASIC) and rubascal (Pascal/Ruby)

[runarorama]

Stacked on top of #6253 (base branch: pluggable-syntax-dialects).

Two more surface syntaxes over the same content-addressed AST — added "for shits and giggles" — both round-tripping through the Surface IR like the existing alt dialects:

basison — BASIC-flavored

FUNCTION name(p, …) … RETURN e END FUNCTION, SELECT CASE … END SELECT, IF c THEN t ELSE e END IF, LET bindings, AS for type ascription, UPPERCASE keywords, : statement separator. The full type rides on a preceding name AS Type signature line.

gcd AS Nat -> Nat -> Nat
FUNCTION gcd(a, b)
  go AS Nat -> Nat -> Nat
  FUNCTION go(x, y)
    RETURN IF y == 0 THEN x ELSE go(y, Nat.mod(x, y)) END IF
  END FUNCTION
  RETURN go(a, b)
END FUNCTION

rubascal — a Pascal/Ruby hybrid

def name(p, …) … end with an implicit return (the last expression is the result, Ruby-style), case … when p then body … end, if c then t else e end, := value bindings (Pascal), : type ascription, begin … end blocks, ->() lambdas, ; separators, bare end enders.

gcd : Nat -> Nat -> Nat
def gcd(a, b)
  go : Nat -> Nat -> Nat
  def go(x, y)
    if y == 0 then x else go(y, Nat.mod(x, y)) end
  end
  go(a, b)
end

Both are registered in Unison.Syntax.Dialect and selectable with syntax.set basison / syntax.set rubascal. The dialects transcript gains a demo section for each, and view/edit round-trip to identical hashes (verified on functions, recursion, tuples, pattern matching, documented definitions, and typed local helpers).