Add migration code to player skins using '.' delimiters (#105)

Previously, the players would have their selected skin reset after renaming
the skin textures to the dot separator. This commit implements skin name
migration to ease the transition for server owners.

See 'skins.__fuzzy_match_skin_name' for a detailed explanation.

unittest.lua

@@ -0,0 +1,50 @@
+local function get_skin(skin_name)
+	local skin = skins.get(skin_name)
+		or skins.__fuzzy_match_skin_name("(unittest)", skin_name, true)
+	return skin and skin:get_key() or nil
+end
+
+local function run_unittest()
+	local PATH = ":UNITTEST:"
+
+	-- -----
+	-- `.`: Simple register + retrieve operations
+	skins.register_skin(PATH, "player.DotSep.png")
+	skins.register_skin(PATH, "player._DotSep_666_.1.png")
+
+	assert(get_skin("player.DotSep"))
+	assert(get_skin("player._DotSep_666_.1"))
+	assert(get_skin("player.DotSep.1") == nil)
+
+	-- -----
+	-- Ambiguous skin names (filenames without extension). Register + retrieve
+	skins.new("player_AmbSki")
+	skins.new("player_AmbSki_1")
+	skins.new("player_AmbSki_666_1")
+
+	assert(get_skin("player_AmbSki"))
+	assert(get_skin("player_AmbSki_") == nil)
+	assert(get_skin("player_AmbSki_1"))
+	assert(get_skin("player_AmbSki_666_1"))
+	-- There are no `__` patterns as they were silently removed by string.split
+
+
+	-- -----
+	-- Mod Storage backwards compatibility
+	-- Match the old `_` notation to `.`-separated skins
+	skins.register_skin(PATH, "player.ComPat42.png")
+	skins.register_skin(PATH, "player.ComPat42.5.png")
+	skins.register_skin(PATH, "player._Com_Pat_42.png")
+	skins.register_skin(PATH, "player._Com_Pat_42.1.png")
+
+	assert(get_skin("player_ComPat42") == "player.ComPat42")
+	assert(get_skin("player_ComPat42_5") == "player.ComPat42.5")
+	assert(get_skin("player_Com_Pat_42") == "player._Com_Pat_42")
+	assert(get_skin("player_Com_Pat_42_1") == "player._Com_Pat_42.1")
+
+
+	error("Unittest passed! Please disable them now.")
+end
+
+run_unittest()
+