Refactor API for 2.0.0 release

[passsy]

Official Dart 3 support, breaking changes, and new Alphabet API.

Breaking changes:

• nanoid(10) is now nanoid(length: 10). The length parameter has been changed from an optional positional to an optional named parameter.
• The async API import 'package:nanoid/async.dart'; has been removed. Call await Future(() {}); or before calling nanoid() to achieve the same effect.
• Changed package name from import 'package:nanoid/nanoid.dart'; to import 'package:nanoid2/nanoid2.dart';
• The insecure API import 'package:nanoid/non-secure.dart'; has been removed. Call nanoid(random: Random()); instead to force a non-secure (and faster) random number generator.
  nonoid() automatically uses Random() instead of Random.secure() on platforms that do not provide a cryptographically secure source of random numbers.
• Removed the customAlphabet('custom', 10) API. Use nanoid(alphabet: 'custom', length: 10); instead.

New APIs:

• The Alphabet class provides a set of predefined alphabets for various use cases.

Inspired by pd4d10/nanoid-dart#5

[codecov]

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