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]

What a "word" is when talking about sizes is confusing because it's a
highly overloaded term. Intel, Microsoft, and GDB are just a few vendors
that have their own definition of what a "word" is. Specs that used the
"wordsize" guard actually were mostly testing for the size of the C
`long` fundamental type, so rename the guard for clarity.

Also, get the size of `long` directly from RbConfig instead of guessing
using Integer#size. Integer#size is not guaranteed to have anything to
do with the `long` type.