Implement $VAR expansion in commands (issue #25)

Extend read_command_body() to detect $NAME and trigger interpolation
using the same fragment-split mechanism as ${expr}. The lexer collects
the identifier into cmd_envvar; the parser's unified parseinterp()
branches on it to emit tostring(getenv(NAME)).

llex.h

@@ -80,6 +80,7 @@ typedef struct LexState {
   TString *envn;  /* environment variable name */
   TString *brkn;  /* "break" name (used as a label) */
   TString *glbn;  /* "global" name (when not a reserved word) */
+  TString *cmd_envvar;       /* envvar name for $NAME in commands */
   lu_byte cmd_mode;  /* 0=normal, 1=backtick command, 2=interactive command */
   lu_byte saved_cmd_mode;  /* cmd_mode to restore after interpolation */
 } LexState;