Implement RFC 0028: calling untyped R functions

[fabriceHategekimana]

Tracking issue for implementing rfcs/0028-calling-untyped-r-functions.md
(accepted in #28).

Scope

• Lang::RFunction with parameters p₁…pₙ typed as (Any, …, Any) -> Any
  with n parameters, instead of Type::UnknownFunction with none.
• Applying it checks arity only (parameters.len()), argument types
  unchecked, result type Any.
• Preloaded untyped builtins (functions_R.txt) keep Type::UnknownFunction,
  but it becomes variadic: any number of arguments, returns Any.
• New error message for arity mismatch on an untyped R function, replacing the
  current confusing "no signature accepts these arguments" message.

Checklist (from the RFC)

• cases/ entry: untyped function(a, b) defined, called, and cast
• cases/ entry: arity error message
• cases/ entry: preloaded builtin called with arguments (Position(1, 2))
• syntaxe.md updated in both copies (this repo and typr.github.io)
• docs/philosophy/intro.md and docs/reference/escape-hatches.md updated
  on typr.github.io; the noplayground marker on the philosophy block
  removed
• Implemented in: filled in on the RFC header once shipped