Unify command tokens: remove TK_COMMAND_INTERP and TK_INTERACTIVE_INTERP

Use cmd_mode (already on LexState) to signal whether interpolation
follows, instead of separate *_INTERP token variants. This eliminates
duplicate simple/interpolated code paths in the parser — commandexp()
and interactivestat() now each use a single unified loop that checks
cmd_mode != 0. A shared parseinterp() helper handles each ${expr}
interpolation cycle.

llex.h

@@ -41,10 +41,8 @@ enum RESERVED {
   TK_DBCOLON, TK_EOS,
   TK_FLT, TK_INT, TK_NAME, TK_STRING,
   TK_COMMAND,
-  TK_COMMAND_INTERP,
   TK_ENVVAR,
-  TK_INTERACTIVE,
-  TK_INTERACTIVE_INTERP
+  TK_INTERACTIVE
 };
 
 /* number of reserved words */