file_icons: Associate .R files with R (#24907)

Closes #24899

Sorted using:
https://github.com/zed-industries/zed/pull/12487#issue-2325944879

Release Notes:

- N/A

Co-authored-by: Marshall Bowers <git@maxdeviant.com>

assets/icons/file_icons/file_types.json

@@ -10,6 +10,7 @@
     "Emakefile": "erlang",
     "MERGE_MSG": "vcs",
     "NOTES_EDITMSG": "vcs",
+    "R": "r",
     "TAG_EDITMSG": "vcs",
     "aac": "audio",
     "accdb": "storage",