`dayjs.tz()` needs to have timezone parameter as optional #1079

Describe the bug Because you can set a default timezone, the dayjs.tz() method should have the second parameter (timezone) as optional. It works fine in JS — just missing the correct types for TS.

Expected behavior

1. Set dayjs.tz.setDefault("America/Phoenix");
2. dayjs.tz('some valid date') <- that shouldn't throw a TypeScript error
3. dayjs('some valid date').tz() <- that shouldn't throw a TypeScript error either

Potential Solution

declare module 'dayjs' {
  interface Dayjs {
    tz(timezone?: string): Dayjs;
  }

  interface DayjsTimezone {
    (date?: ConfigType, timezone?: string): Dayjs;
  }
}

Information

• Day.js Version: v1.8.36
• OS: macOS v10.15.6
• Browser: Firefox Developer Edition v82.0b2
• Time zone: America/Phoenix