patch 8.2.1584: Vim9: cannot use "true" for "skipstart" in prop_find()

Problem:    Vim9: cannot use "true" for "skipstart" in prop_find().
Solution:   Use dict_get_bool() instead of tv_get_number(). (closes #6852)
diff --git a/src/textprop.c b/src/textprop.c
index 9dff6b8..beb9a27 100644
--- a/src/textprop.c
+++ b/src/textprop.c
@@ -645,9 +645,7 @@
 	return;
     }
 
-    di = dict_find(dict, (char_u *)"skipstart", -1);
-    if (di != NULL)
-	skipstart = tv_get_number(&di->di_tv);
+    skipstart = dict_get_bool(dict, (char_u *)"skipstart", 0);
 
     if (dict_find(dict, (char_u *)"id", -1) != NULL)
 	id = dict_get_number(dict, (char_u *)"id");