Fix T85051: Add bisect distance as a parameter to the mirror modifier

The `bisect_distance` in the mirror modifier was hard-coded to `0.001`.
This would result in some unexpected behavior like vertices close
to the mirror plane being deleted or merged.

The fix now adds a parameter to the mirror modifier to expose the
bisect distance to the user. The default is set to the previous
hard-coded value to not "change" previous files.

Ref D10201

source/blender/modifiers/intern/MOD_mirror.c

@@ -165,6 +165,13 @@ static void panel_draw(const bContext *UNUSED(C), Panel *panel)
   uiLayoutSetActive(sub, RNA_boolean_get(ptr, "use_mirror_merge"));
   uiItemR(sub, ptr, "merge_threshold", 0, "", ICON_NONE);
 
+  bool is_bisect_set[3];
+  RNA_boolean_get_array(ptr, "use_bisect_axis", is_bisect_set);
+
+  sub = uiLayoutRow(col, true);
+  uiLayoutSetActive(sub, is_bisect_set[0] || is_bisect_set[1] || is_bisect_set[2]);
+  uiItemR(sub, ptr, "bisect_threshold", 0, IFACE_("Bisect Distance"), ICON_NONE);
+
   modifier_panel_end(layout, ptr);
 }