Compile backtick commands as __command() calls with ${} interpolation

Backtick expressions now compile as calls to a global __command()
function instead of being treated as plain strings. Interpolation
via ${expr} is supported, with each interpolated value wrapped in
tostring() for type safety. Commands are parsed in primaryexp so
suffix operations like `.stdout` work directly.

Adds a stub __command that returns {code, stdout, stderr} for testing
until the real implementation (issue #03).

llex.h

@@ -39,7 +39,9 @@ enum RESERVED {
   TK_IDIV, TK_CONCAT, TK_DOTS, TK_EQ, TK_GE, TK_LE, TK_NE,
   TK_SHL, TK_SHR,
   TK_DBCOLON, TK_EOS,
-  TK_FLT, TK_INT, TK_NAME, TK_STRING
+  TK_FLT, TK_INT, TK_NAME, TK_STRING,
+  TK_COMMAND,
+  TK_COMMAND_INTERP
 };
 
 /* number of reserved words */
@@ -77,6 +79,7 @@ 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) */
+  lu_byte in_command;  /* nonzero when continuing a backtick string after ${} */
 } LexState;
 
 
@@ -88,6 +91,7 @@ LUAI_FUNC void luaX_next (LexState *ls);
 LUAI_FUNC int luaX_lookahead (LexState *ls);
 LUAI_FUNC l_noret luaX_syntaxerror (LexState *ls, const char *s);
 LUAI_FUNC const char *luaX_token2str (LexState *ls, int token);
+LUAI_FUNC int luaX_readcommandcont (LexState *ls);
 
 
 #endif