New: Ruby 3.1.0

Here's a port for the recently released Ruby 3.1.0. Major improvements
are:

* In-process JIT compiler (which respects W^X)
* New debugger
* Improved error reporting
* Autocomplete in irb shell

Release notes at:
https://www.ruby-lang.org/en/news/2021/12/25/ruby-3-1-0-released/

Diff below to allow building ruby31 packages for ports.

Tested on amd64. OK to import?

Thanks,
Jeremy

Index: ruby.port.mk
===================================================================
RCS file: /cvs/ports/lang/ruby/ruby.port.mk,v
retrieving revision 1.105
diff -u -p -r1.105 ruby.port.mk
--- ruby.port.mk 6 Oct 2021 21:10:19 -0000 1.105
+++ ruby.port.mk 9 Nov 2021 12:36:46 -0000
@@ -24,7 +24,7 @@ MODRUBY_HANDLE_FLAVORS ?= No
# If ruby.pork.mk should handle FLAVORs, define a separate FLAVOR
# for each ruby interpreter
. if !defined(FLAVORS)
-FLAVORS= ruby27 ruby30
+FLAVORS= ruby27 ruby30 ruby31
. if !${CONFIGURE_STYLE:L:Mext}
FLAVORS+= jruby
. endif
@@ -52,7 +52,7 @@ FLAVOR = ruby30

# Check for conflicting FLAVORs and set MODRUBY_REV appropriately based
# on the FLAVOR.
-. for i in ruby27 ruby30 jruby
+. for i in ruby27 ruby30 ruby31 jruby
. if ${FLAVOR:M$i}
MODRUBY_REV = ${i:C/ruby([0-9])/\1./}
. if ${FLAVOR:N$i:Mruby27} || \