v2.0.0

• Internals are reworked: now package is relying on a tree-like structure to improve maintainability and scalability.
• Fixed indentSize validation (only values >=1 are allowed).
• Resolved #6:

# BEFORE:
field:
  - 
    - 5
    - 2
  - 
    a: 3
    b: 6

# AFTER:
field:
  - - 5
    - 2
  - a: 3
    b: 6

• Update min Dart SDK version constraint from 2.12.0 to 3.0.0
• Update lints: ^2.0.1 to lints: ^3.0.0

BREAKING CHANGES:

• YAMLWriter renamed to YamlWriter (Effective Dart reference).
• Deprecated identSize property is removed. Use indentSize instead.
• toEncodable property is removed. Use constructor parameter instead (YamlWriter#toEncodable).