feat: add import of commands + small fixes related to command options

[carafelix]

closes #38

  const sayHi = new Command("hi","say hi", (ctx) => ctx.reply("hi"))
  const sayBye = new Command("bye","say bye", (ctx) => ctx.reply("bye"), { ignoreCase: true })
  const myCmds = new CommandGroup().add(sayHi).add(sayBye)

fix: merge options instead of complete override when using the .command creation method.

• when passing a options param to the CommandGroup.command(...) method, the default options from the command group now merges with the specific options of that command, which ofc takes priority. Before the object was being overridden completely, ignoring the default group options. https://github.com/grammyjs/commands/blob/0f92ae16f08a1e8e991156eebd2c2fd2823f8c11/src/command-group.ts#L141-L143

feat: prevent whitespace only custom prefixes.

• improve the prevention for using whitespace only custom prefixes. Before it was checking only against prefix === "" https://github.com/grammyjs/commands/blob/0f92ae16f08a1e8e991156eebd2c2fd2823f8c11/src/command-group.ts#L78-L79 https://github.com/grammyjs/commands/blob/0f92ae16f08a1e8e991156eebd2c2fd2823f8c11/src/command.ts#L120