Add: Setting to control minimum distance between towns. (#14893)

Backported from JGRPP.

src/table/settings/economy_settings.ini

@@ -249,6 +249,17 @@ str      = STR_CONFIG_SETTING_CITY_SIZE_MULTIPLIER
 strhelp  = STR_CONFIG_SETTING_CITY_SIZE_MULTIPLIER_HELPTEXT
 strval   = STR_JUST_COMMA
 
+[SDT_VAR]
+var      = economy.town_min_distance
+type     = SLE_UINT16
+def      = 20
+min      = 10
+max      = 500
+interval = 5
+str      = STR_CONFIG_SETTING_TOWN_MIN_DISTANCE
+strhelp  = STR_CONFIG_SETTING_TOWN_MIN_DISTANCE_HELPTEXT
+strval   = STR_JUST_INT
+
 [SDT_BOOL]
 var      = economy.mod_road_rebuild
 from     = SLV_77