Add test case for non-expansion in string variables

regression/dollar_in_player_input.alan

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+--==============================================================================
+-- BUG: String parameters may contain Alan special $ symbols, which can lead to
+--      infinite recursion when the player types "$1".
+--==============================================================================
+
+THE room IsA LOCATION
+  Has text "".
+  VERB write
+    DOES
+      "You write ""$1""."
+      Set text of room to str.
+  END VERB write.
+END THE room.
+
+-- Verb Write
+SYNTAX write = write (str)
+  WHERE str IsA string
+    ELSE "You can only use strings with this verb."
+
+
+-- Verb Read
+SYNTAX read = read.
+
+VERB read
+  DOES
+    Say text of room.
+END VERB read.
+
+Start at room.

regression/dollar_in_player_input.expected

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+########## dollar_in_player_input ##########
+
+        No warnings or errors detected.
+
+
+
+Room
+
+> read
+
+> write "Hello world."
+You write "Hello world.".
+
+> read
+Hello world.
+
+> write "Hello.$pWorld"
+You write "Hello.$pWorld".
+
+> read
+Hello.$pWorld
+
+> write "Hello.$1"
+You write "Hello.$1".
+
+> read
+Hello.$1
+
+> write "$1"
+You write "$1".
+
+> 
+
+Do you want to UNDO, RESTART, RESTORE or QUIT ? 

regression/dollar_in_player_input.input

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+read
+write "Hello world."
+read
+write "Hello.$pWorld"
+read
+write "Hello.$1"
+read
+write "$1"