Implement prefix-based interactive commands via ! (issue #14)

Add !command syntax that runs commands with inherited terminal (no
capture). The lexer treats ! as a statement-starting token, reading
to end-of-line with the same interpolation/escape/pipe support as
backtick commands. The C function sets _ = {code=N, stdout="", stderr=""}.

llex.h

@@ -42,7 +42,9 @@ enum RESERVED {
   TK_FLT, TK_INT, TK_NAME, TK_STRING,
   TK_COMMAND,
   TK_COMMAND_INTERP,
-  TK_ENVVAR
+  TK_ENVVAR,
+  TK_INTERACTIVE,
+  TK_INTERACTIVE_INTERP
 };
 
 /* number of reserved words */
@@ -80,7 +82,7 @@ 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 ${} */
+  lu_byte cmd_mode;  /* 0=normal, 1=backtick command, 2=interactive command */
 } LexState;