allow funcname which starts with underscore

Author: tyru

autoload/vimlparser.vim

@@ -1339,7 +1339,7 @@ function! s:VimLParser.parse_cmd_function()
   if left.type == s:NODE_IDENTIFIER
     let s = left.value
     let ss = split(s, '\zs')
-    if ss[0] !=# '<' && !s:isupper(ss[0]) && stridx(s, ':') == -1 && stridx(s, '#') == -1
+    if ss[0] !=# '<' && ss[0] !=# '_' && !s:isupper(ss[0]) && stridx(s, ':') == -1 && stridx(s, '#') == -1
       throw s:Err(printf('E128: Function name must start with a capital or contain a colon: %s', s), left.pos)
     endif
   endif

test/test_funcname.ok

@@ -0,0 +1,3 @@
+(function (_F))
+(function (s:SID))
+(function (<SID>SID))

test/test_funcname.vim

@@ -0,0 +1,8 @@
+function _F()
+endfunction
+
+function s:SID()
+endfunction
+
+function <SID>SID()
+endfunction