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=""}.

linit.c

@@ -71,6 +71,8 @@ static int luaB_setenv (lua_State *L) {
 static void opencommand (lua_State *L) {
   lua_pushcfunction(L, luaB_command);
   lua_setglobal(L, "__command");
+  lua_pushcfunction(L, luaB_interactive);
+  lua_setglobal(L, "__interactive");
   lua_pushcfunction(L, luaB_getenv);
   lua_setglobal(L, "__getenv");
   lua_pushcfunction(L, luaB_setenv);