patch 9.1.0425: filetype: purescript files are not recognized

Problem:  filetype: purescript files are not recognized
Solution: recognize '*.purs' files as purescript filetype,
          include basic purescript filetype plugin
          (Riley Bruins)

Reference: https://github.com/purescript/documentation/blob/master/language/Syntax.md#comments

closes: #14813

Signed-off-by: Riley Bruins <ribru17@hotmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/ftplugin/purescript.vim b/runtime/ftplugin/purescript.vim
new file mode 100644
index 0000000..b3300bb
--- /dev/null
+++ b/runtime/ftplugin/purescript.vim
@@ -0,0 +1,14 @@
+" Vim filetype plugin
+" Language:	purescript
+" Maintainer:	Riley Bruins <ribru17@gmail.com>
+" Last Change:	2024 May 19
+
+if exists('b:did_ftplugin')
+  finish
+endif
+let b:did_ftplugin = 1
+
+setl comments=s1f:{-,mb:\ ,ex:-},:--\ \|,:--
+setl commentstring=--\ %s
+
+let b:undo_ftplugin = 'setl com< cms<'