runtime(vim): Update base-syntax, improve folding function matches

- Allow function command modifiers.
- Match function bodies starting with empty lines.

Command modifiers reported by @Konfekt.

fixes #15671
closes: #15674

Signed-off-by: Doug Kearns <dougkearns@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/syntax/testdir/input/vim_ex_def_fold.vim

@@ -1,5 +1,6 @@
 " Vim :def command
-" VIM_TEST_SETUP let g:vimsyn_folding = "f" | set fdm=syntax
+" VIM_TEST_SETUP let g:vimsyn_folding = "f"
+" VIM_TEST_SETUP setl fdc=2 fdl=999 fdm=syntax
 
 
 " list
@@ -109,6 +110,18 @@ def Foo()
 enddef
 
 
+" command modifiers
+
+silent! def Foo()
+enddef
+
+
+" leading command separator
+
+echo "Foo" | def Foo()
+enddef
+
+
 " fold-region ending
 
 def Foo()
@@ -128,3 +141,45 @@ enddef
 :def Foo()
 :enddef
 
+
+" Issue #15671
+" No recognition of :fun or :def bodies commencing with empty lines if
+" g:vimsyn_folding contains "f"
+
+def MA1()
+
+
+    return
+enddef
+
+def MA2()
+    return
+enddef
+
+def MB1(): void
+
+    return
+enddef
+
+def MB2(): void
+    return
+enddef
+
+def MC1(_: any)
+
+    return
+enddef
+
+def MC2(_: any)
+    return
+enddef
+
+def MD1(_: any): void
+
+    return
+enddef
+
+def MD2(_: any): void
+    return
+enddef
+