Official Dart lint rules applied with recommend set

BETA version 0.6.0 introduced custom lint rules, see Lint rules and analysis options updated #8 from geospatial repo.

However since that the Dart team has announced new official lint rules:

• Official Dart lint rules
• Customizing static analysis
• Official Flutter lint rules

In general, it's better to follow recommend lints to keep code consistent with other developers.

Steps to configure official lints in old package

1. First apply dart pub add --dev lints that adds lints: ^1.0.1 or other version to dev_dependencies
2. Then edit analysis_options.yaml by adding include: package:lints/recommended.yaml
3. Customize analysis_options.yaml

Modified lint rules and analysis_options.yaml as follows - testing these:

# Analysis options / lint rules (2021-07-10)
# 
# Documentation (official for Dart)
#     https://pub.dev/packages/lints
#     https://dart.dev/guides/language/analysis-options
#     https://dart-lang.github.io/linter/lints/
#
# Documentation (official for Flutter)
#     https://pub.dev/packages/flutter_lints
#
# Getting started (before new official Dart and Flutter lints announced!)
#     https://dash-overflow.net/articles/getting_started/
#         Some of comments originated from this blog.
#     https://rydmike.com/blog => My Flutter Linting Preferences (Jan 10, 2021)
#         Some of comments originated from this blog.

## Include recommended set of the Official Dart lint rules.
include: package:lints/recommended.yaml

analyzer:
  exclude:
    # Ignore warnings of files generated by json_serializable, built_value etc.
    - "**/*.g.dart"

    # Ignore warnings of files generated by Freezed.
    - "**/*.freezed.dart"

  language:
    # See => https://github.com/dart-lang/language/blob/master/resources/type-system/strict-raw-types.md
    strict-raw-types: true

  strong-mode:
    # Explained best here => https://dash-overflow.net/articles/getting_started/
    implicit-casts: false

    # Explained best here => https://dash-overflow.net/articles/getting_started/
    implicit-dynamic: false

Implemented in BETA-version 0.7.0