feat: add pandoc-crossref plugin (#997)

Co-authored-by: Daiki Noda <sys9kdr@noreply.github.com>

Author: sys9kdr

README.md

@@ -560,6 +560,7 @@ The `asdf` core provides a [security policy](https://github.com/asdf-vm/asdf/sec
 | pachctl                       | [abatilo/asdf-pachctl](https://github.com/abatilo/asdf-pachctl)                                                   |
 | Packer                        | [asdf-community/asdf-hashicorp](https://github.com/asdf-community/asdf-hashicorp)                                 |
 | Pandoc                        | [Fbrisset/asdf-pandoc](https://github.com/Fbrisset/asdf-pandoc)                                                   |
+| pandoc-crossref               | [sys9kdr/asdf-pandoc-crossref](https://github.com/sys9kdr/asdf-pandoc-crossref)                                   |
 | patat                         | [airtonix/asdf-patat](https://github.com/airtonix/asdf-patat)                                                     |
 | peco                          | [asdf-community/asdf-peco](https://github.com/asdf-community/asdf-peco)                                           |
 | pdm                           | [1oglop1/asdf-pdm](https://github.com/1oglop1/asdf-pdm)                                                           |

plugins/pandoc-crossref

@@ -0,0 +1 @@
+repository = https://github.com/sys9kdr/asdf-pandoc-crossref