[Feature #20205]

The warning now suggests running with --debug-frozen-string-literal:

```
test.rb:3: warning: literal string will be frozen in the future (run with --debug-frozen-string-literal for more information)
```

When using --debug-frozen-string-literal, the location where the string
was created is shown:

```
test.rb:3: warning: literal string will be frozen in the future
test.rb:1: info: the string was created here
```

When resurrecting strings and debug mode is not enabled, the overhead is a simple FL_TEST_RAW.
When mutating chilled strings and deprecation warnings are not enabled,
the overhead is a simple warning category enabled check.

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

[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>

Since these files are written in a wide character encoding, stop at
first NUL byte and are actually empty.  ASCII-incompatible encodings
have never been supported as source encoding.