refactor: Fix multiple analysis issues

[alverone]

Description

This pull request aims to fix some of the analysis and other issues without changing the functionality and/or introducing breaking changes.

Change log:

• changed the usage of linter rules from lints to flutter_lints, which are more suitable for Flutter packages;
• enabled or added multiple rules for both the package and example projects; see the list in the details section;
• removed default, commented sections from analysis options and pubspec files that came with Flutter project setup;
• updated documentation for the majority of classes:
  • fixed grammar, typos, and misspellings;
  • simplified descriptions of parameters and methods to be more concise;
  • fixed broken documentation references wherever applicable ([reference to class]);
• in both the LinkViewHorizontal and the LinkViewVertical widgets:
  • changed the parameter showMultiMedia to be non-null since the potential casting of a nullable type could lead to runtime issues;
  • added missing assignment of border-radius to the parent InkWell widgets, which led to visible rectangular corners on tap;
• in the LinkViewVertical widget, fixed the border-radius parameter to use the widget parameter instead of a constant value of 12;
• in the AnyLinkPreview widget:
  • updated documentation for widget parameters;
  • updated some parameters and methods to have exact input and return types;
  • moved the call to super.initState() in the overridden initState method to the top [as required by framework](https://api.flutter.dev/flutter/widgets/State/initState.html#:~:text=Implementations%20of%20this%20method%20should%20start%20with%20a%20call%20to%20the%20inherited%20method%2C%20as%20in%20super.initState().);
  • made some private state variables late and final if they were never changed after initState;
• fixed analysis info messages wherever applicable;
• updated example:
  • removed obsolete and/or unused code;
  • extracted custom preview widget that uses the AnyLinkPreview.builder() constructor into a separate widget for simplification;
  • removed broken tests that came with the default Flutter project;
• updated README.md.

Details

Enabled/added rules:

    sized_box_for_whitespace
    prefer_const_constructors
    comment_references
    avoid_multiple_declarations_per_line
    noop_primitive_operations
    cascade_invocations
    avoid_redundant_argument_values
    lines_longer_than_80_chars
    prefer_int_literals
    prefer_const_constructors_in_immutables
    avoid_void_async
    require_trailing_commas

Screenshots of the example app (before & after):

Screenshots:

#### After: *** #### Before: