Add Validations for equals() and not_equals()

[MGatner]

Description
Per conversation in Slack, there is currently no way to check a precise value (or not), such as confirming a user typed "yes". This adds validation rules for equals and not_equals, along with their language texts, user guide entires, and tests.

Checklist:

• Securely signed commits
• Component(s) with PHPdocs
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[atishhamte]

Please provide the unit test case related to all the datatype and string case sensitive and insensitive comparison.
Also update the user guide as well.

[nowackipawel]

Could not we just use in_list / not_in_list / regex rules ?

[MGatner]

@nowackipawel While those supply overlapping functionality, they make for weird error messages such as "The confirmation field must be one of: yes." This allows for clearer messages as well as easier to understand code.

[MGatner]

Please provide the unit test case related to all the datatype and string case sensitive and insensitive comparison.
Also update the user guide as well.

I accidentally sent user guide and tests to my develop branch. I included those and added tests for case sensitivity. Data types are irrelevant because rules "must accept a string as the first parameter" (https://codeigniter4.github.io/CodeIgniter4/libraries/validation.html#creating-custom-rules).