patch 8.2.0110: prop_find() is not implemented

Problem:    prop_find() is not implemented.
Solution:   Implement prop_find(). (Ryan Hackett, closes #5421, closes #4970)
diff --git a/src/proto/textprop.pro b/src/proto/textprop.pro
index 028e3d4..30a846e 100644
--- a/src/proto/textprop.pro
+++ b/src/proto/textprop.pro
@@ -6,6 +6,7 @@
 int find_visible_prop(win_T *wp, int type_id, int id, textprop_T *prop, linenr_T *found_lnum);
 proptype_T *text_prop_type_by_id(buf_T *buf, int id);
 void f_prop_clear(typval_T *argvars, typval_T *rettv);
+void f_prop_find(typval_T *argvars, typval_T *rettv);
 void f_prop_list(typval_T *argvars, typval_T *rettv);
 void f_prop_remove(typval_T *argvars, typval_T *rettv);
 void f_prop_type_add(typval_T *argvars, typval_T *rettv);