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_function_fold.vim

@@ -1,5 +1,6 @@
 " Vim :function 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
@@ -174,6 +175,18 @@ function Foo()
 endfunction
 
 
+" command modifiers
+
+silent! function Foo()
+endfunction
+
+
+" leading command separator
+
+echo "Foo" | function Foo()
+endfunction
+
+
 " delete function
 
 delfunction Foo
@@ -220,3 +233,45 @@ endfunction
 :function Foo()
 :endfunction
 
+
+" Issue #15671
+" No recognition of :fun or :def bodies commencing with empty lines if
+" g:vimsyn_folding contains "f"
+
+fun FA1()
+
+
+    return
+endfun
+
+fun FA2()
+    return
+endfun
+
+fun FB1() abort
+
+    return
+endfun
+
+fun FB2() abort
+    return
+endfun
+
+fun FC1(_)
+
+    return
+endfun
+
+fun FC2(_)
+    return
+endfun
+
+fun FD1(_) abort
+
+    return
+endfun
+
+fun FD2(_) abort
+    return
+endfun
+