patch 8.2.0149: maintaining a Vim9 branch separately is more work

Problem:    Maintaining a Vim9 branch separately is more work.
Solution:   Merge the Vim9 script changes.
diff --git a/src/proto/vim9script.pro b/src/proto/vim9script.pro
new file mode 100644
index 0000000..29fa269
--- /dev/null
+++ b/src/proto/vim9script.pro
@@ -0,0 +1,8 @@
+/* vim9script.c */
+int in_vim9script(void);
+void ex_vim9script(exarg_T *eap);
+void ex_export(exarg_T *eap);
+void free_imports(int sid);
+void ex_import(exarg_T *eap);
+char_u *handle_import(char_u *arg_start, garray_T *gap, int sid);
+/* vim: set ft=c : */