patch 9.1.0593: filetype: Asymptote files are not recognized

Problem:  filetype: Asymptote files are not recognized
Solution: detect '*.asy' files as asy filetype, include
          ftplugin and syntax plugin (AvidSeeker).

Reference: https://asymptote.sourceforge.io/

closes: #15252

Signed-off-by: AvidSeeker <avidseeker7@protonmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/ftplugin/asy.vim

@@ -0,0 +1,14 @@
+" Vim filetype plugin
+" Language:     Asymptote
+" Maintainer:	AvidSeeker <avidseeker7@protonmail.com>
+" Last Change:  2024 Jul 13
+"
+
+if exists("b:did_ftplugin")
+  finish
+endif
+let g:did_ftplugin = 1
+
+setlocal commentstring=/*\ %s\ */
+
+let b:undo_ftplugin = "setl commentstring<"