patch 8.1.0281: parsing command modifiers is not separated

Problem:    Parsing command modifiers is not separated.
Solution:   Move command modifier parsing to a separate function.
diff --git a/src/proto/ex_docmd.pro b/src/proto/ex_docmd.pro
index 00800e1..50a2c0c 100644
--- a/src/proto/ex_docmd.pro
+++ b/src/proto/ex_docmd.pro
@@ -4,6 +4,7 @@
 int do_cmdline(char_u *cmdline, char_u *(*fgetline)(int, void *, int), void *cookie, int flags);
 int getline_equal(char_u *(*fgetline)(int, void *, int), void *cookie, char_u *(*func)(int, void *, int));
 void *getline_cookie(char_u *(*fgetline)(int, void *, int), void *cookie);
+int parse_command_modifiers(exarg_T *eap, char_u **errormsg);
 int parse_cmd_address(exarg_T *eap, char_u **errormsg);
 int checkforcmd(char_u **pp, char *cmd, int len);
 int modifier_len(char_u *cmd);