[PATCH] fix underflow

https://bugs.ruby-lang.org/issues/21785#note-10
> It is too late to introduce it in Ruby 4.0, let's aim for 4.1.

This reverts commits:

* d0b72429a93e54f1f956b4aedfc25c57dc7001aa
  Add support for signed and unsigned LEB128 to pack/unpack.

* 68a900e30b4ca1537d7975c3a619fd94fca7b084
  add news for pack / unpack directives

This commit adds a new pack format command `R` and `r` for unsigned and
signed LEB128 encoding.  The "r" mnemonic is because this is a
"vaRiable" length encoding scheme.

LEB128 is used in various formats including DWARF, WebAssembly, MQTT,
and Protobuf.

[Feature #21785]

Instructions for this code:

```ruby
  # frozen_string_literal: true

[a].pack("C")
```

Before this commit:

```
== disasm: #<ISeq:<main>@test.rb:1 (1,0)-(3,13)>
0000 putself                                                          (   3)[Li]
0001 opt_send_without_block                 <calldata!mid:a, argc:0, FCALL|VCALL|ARGS_SIMPLE>
0003 newarray                               1
0005 putobject                              "C"
0007 opt_send_without_block                 <calldata!mid:pack, argc:1, ARGS_SIMPLE>
0009 leave
```

After this commit:

```
== disasm: #<ISeq:<main>@test.rb:1 (1,0)-(3,13)>
0000 putself                                                          (   3)[Li]
0001 opt_send_without_block                 <calldata!mid:a, argc:0, FCALL|VCALL|ARGS_SIMPLE>
0003 putobject                              "C"
0005 opt_newarray_send                      2, :pack
0008 leave
```

Co-authored-by: Maxime Chevalier-Boisvert <maxime.chevalierboisvert@shopify.com>
Co-authored-by: Aaron Patterson <tenderlove@ruby-lang.org>

Fixes a compiler warning about the function unknown_directive:

pack.c:160:1: warning: function 'unknown_directive' could be declared with attribute 'noreturn' [-Wmissing-noreturn]

* [Bug #19150]