examples: support directly loading sokoban level files by path

examples/sokoban/corners.txt

@@ -1,12 +1,12 @@
 Push the boxes to the corners of the warehouse.@It is a lot of work...@Have fun.
 #########################
-#@         #           @#
+#.         #           .#
 #          #            #
 #    ##    ### ####  ####
 #    ##    #    #       #
-#  b #  b  #  b #  b    #
+#  $ #  $  #  $ #  $    #
 #    #     #    #       #
 #    #     #   ###   ####
 #    #     #   ###      #
-#@   #     p           @#
+#.   #     @           .#
 #########################

examples/sokoban/simple.txt

@@ -2,8 +2,8 @@ Move your player to the box and push it.@Use the arrows to move your player.
 ##############
 #            #
 #            #
-#   b        #
-#      @     #
-# p          #
+#   $        #
+#      .     #
+# @          #
 #            #
 ##############

examples/sokoban/sokoban.v

@@ -1,3 +1,4 @@
+import os
 import os.asset
 import gg
 import gx
@@ -55,17 +56,20 @@ fn (mut g Game) parse_level(lnumber int) ! {
 		mut row := []rune{}
 		for x, c in line {
 			match c {
-				`#`, ` `, `@` {
+				`#`, ` ` {
 					row << c
 				}
-				`b` { // a normal box
+				`b`, `$` { // a normal box
 					row << ` `
 					boxes << Pos{x, y}
 				}
-				`p` { // a normal player
+				`p`, `@` { // a normal player
 					row << ` `
 					player = Pos{x, y}
 				}
+				`.` { // storage
+					row << `@`
+				}
 				`B` { // box on storage
 					row << `@`
 					boxes << Pos{x, y}
@@ -247,8 +251,12 @@ fn (mut g Game) iid(name string) !int {
 
 fn main() {
 	mut g := &Game{}
-	all_level_names := asset.read_text('/', '_all_levels.txt')!.split_into_lines()
-	g.levels = all_level_names.map(asset.read_text('/', it)!)
+	if os.args.len > 1 {
+		g.levels = os.args[1..].map(os.read_file(it)!)
+	} else {
+		all_level_names := asset.read_text('/', '_all_levels.txt')!.split_into_lines()
+		g.levels = all_level_names.map(asset.read_text('/', it)!)
+	}
 	g.parse_level(0)!
 	g.ctx = gg.new_context(
 		width:        800

examples/sokoban/three_boxes.txt

@@ -1,10 +1,10 @@
 It seems tricky, but it is not ...@Be careful to not get stuck.
          ###          #
     ###### ######     #
-           #  p #     #
+           #  @ #     #
           ##    #######
-#   bbb   #           #
-#      @   @  @ #     #
+#   $$$   #           #
+#      .   .  . #     #
 #      #   #  ###     #
 #      #      ###     #
    ############       #