patch 9.1.1340: cannot complete :filetype arguments

Problem:  cannot complete :filetype arguments (Phạm Bình An)
Solution: add :filetype ex command completion, add "filetypecmd"
          completion type for getcompletion()

fixes: #17165
closes: #17167

Co-authored-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/usercmd.c b/src/usercmd.c
index ff2c353..fafa97d 100644
--- a/src/usercmd.c
+++ b/src/usercmd.c
@@ -71,6 +71,7 @@
     KEYVALUE_ENTRY(EXPAND_FILES, "file"),
     KEYVALUE_ENTRY(EXPAND_FILES_IN_PATH, "file_in_path"),
     KEYVALUE_ENTRY(EXPAND_FILETYPE, "filetype"),
+    KEYVALUE_ENTRY(EXPAND_FILETYPECMD, "filetypecmd"),
     KEYVALUE_ENTRY(EXPAND_FUNCTIONS, "function"),
     KEYVALUE_ENTRY(EXPAND_HELP, "help"),
     KEYVALUE_ENTRY(EXPAND_HIGHLIGHT, "highlight"),