Merge pull request #2900 from tokorom/vim-syntax-fallthrought

[vim] highlight `fallthrough`

Author: gribozavr

utils/vim/syntax/swift.vim

@@ -17,6 +17,7 @@ syn keyword swiftKeyword
       \ defer
       \ do
       \ else
+      \ fallthrough
       \ for
       \ guard
       \ if