feat(rvm): add `rb31` shortcut to use ruby 3.1 (#10745)

plugins/rvm/README.md

@@ -23,6 +23,7 @@ plugins=(... rvm)
 | `rb26`       | `rvm use ruby-2.6`   |
 | `rb27`       | `rvm use ruby-2.7`   |
 | `rb30`       | `rvm use ruby-3.0`   |
+| `rb31`       | `rvm use ruby-3.1`   |
 | `rvm-update` | `rvm get head`       |
 | `gems`       | `gem list`           |
 | `rvms`       | `rvm gemset`         |

plugins/rvm/rvm.plugin.zsh

@@ -26,6 +26,7 @@ rubies=(
   26  'ruby-2.6'
   27  'ruby-2.7'
   30  'ruby-3.0'
+  31  'ruby-3.1'
 )
 
 for v in ${(k)rubies}; do