Implement chilled strings

[Feature #20205]

As a path toward enabling frozen string literals by default in the future,
this commit introduce "chilled strings". From a user perspective chilled
strings pretend to be frozen, but on the first attempt to mutate them,
they lose their frozen status and emit a warning rather than to raise a
`FrozenError`.

Implementation wise, `rb_compile_option_struct.frozen_string_literal` is
no longer a boolean but a tri-state of `enabled/disabled/unset`.

When code is compiled with frozen string literals neither explictly enabled
or disabled, string literals are compiled with a new `putchilledstring`
instruction. This instruction is identical to `putstring` except it marks
the String with the `STR_CHILLED (FL_USER3)` and `FL_FREEZE` flags.

Chilled strings have the `FL_FREEZE` flag as to minimize the need to check
for chilled strings across the codebase, and to improve compatibility with
C extensions.

Notes:
  - `String#freeze`: clears the chilled flag.
  - `String#-@`: acts as if the string was mutable.
  - `String#+@`: acts as if the string was mutable.
  - `String#clone`: copies the chilled flag.

Co-authored-by: Jean Boussier <byroot@ruby-lang.org>

error.c

@@ -3860,6 +3860,12 @@ void
 rb_error_frozen_object(VALUE frozen_obj)
 {
     rb_yjit_lazy_push_frame(GET_EC()->cfp->pc);
+
+    if (CHILLED_STRING_P(frozen_obj)) {
+        CHILLED_STRING_MUTATED(frozen_obj);
+        return;
+    }
+
     VALUE debug_info;
     const ID created_info = id_debug_created_info;
     VALUE mesg = rb_sprintf("can't modify frozen %"PRIsVALUE": ",