reduce number of gsub calls for text variable substitutions

Use lua feature that allows to specify table with substitutions to
string.gsub()

functions.lua

@@ -3112,11 +3112,12 @@ end
 -- replace some variables in the text the NPC speaks and which the player can use to reply
 -- pname: the name of the player that is talking to the NPC
 yl_speak_up.replace_vars_in_text = function(text, dialog, pname)
-	-- Note: the $ char is a special one. It needs to be escaped with %$ in lua.
+	local subs = {
-	text = string.gsub(text, "%$MY_NAME%$", dialog.n_npc)
+		MY_NAME = dialog.n_npc,
-	text = string.gsub(text, "%$NPC_NAME%$", dialog.n_npc)
+		NPC_NAME = dialog.n_npc,
-	text = string.gsub(text, "%$OWNER_NAME%$", dialog.npc_owner)
+		OWNER_NAME = dialog.npc_owner,
-	text = string.gsub(text, "%$PLAYER_NAME%$", pname)
+		PLAYER_NAME = pname,
+	}
 
 	local day_time_name = "day"
 	local day_time = minetest.get_timeofday()
@@ -3127,8 +3128,15 @@ yl_speak_up.replace_vars_in_text = function(text, dialog, pname)
 	else
 		day_time_name = "evening"
 	end
-	text = string.gsub(text, "%$GOOD_DAY%$", "Good "..day_time_name)
+	subs.GOOD_DAY = "Good "..day_time_name
-	text = string.gsub(text, "%$good_DAY%$", "good "..day_time_name)
+	subs.good_DAY = "good "..day_time_name
+
+	-- Note: the $ char is a special one. It needs to be escaped with %$ in lua.
+	-- Note: when substitution argument is a table, we look up
+	-- substitutions in it using substring captured by "()" in
+	-- pattern. "[%a_]+" means one or more letter or underscore.
+	-- If lookup returns nil, then no substitution is made.
+	text = string.gsub(text, "%$([%a_]+)%$", subs)
 
 	return text
 end