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=""}.

lcmd.h

@@ -10,5 +10,6 @@
 #include "lua.h"
 
 int luaB_command (lua_State *L);
+int luaB_interactive (lua_State *L);
 
 #endif