patch 8.2.0491: cannot recognize a <script> mapping using maparg()

Problem:    Cannot recognize a <script> mapping using maparg().
Solution:   Add the "script" key. (closes #5873)
diff --git a/src/map.c b/src/map.c
index 948b32c..9bc9abc 100644
--- a/src/map.c
+++ b/src/map.c
@@ -2200,6 +2200,8 @@
 	dict_add_string(dict, "lhs", lhs);
 	dict_add_string(dict, "rhs", mp->m_orig_str);
 	dict_add_number(dict, "noremap", mp->m_noremap ? 1L : 0L);
+	dict_add_number(dict, "script", mp->m_noremap == REMAP_SCRIPT
+								    ? 1L : 0L);
 	dict_add_number(dict, "expr", mp->m_expr ? 1L : 0L);
 	dict_add_number(dict, "silent", mp->m_silent ? 1L : 0L);
 	dict_add_number(dict, "sid", (long)mp->m_script_ctx.sc_sid);