refactor: replace className() method with addClassName()

[ilandikov]

Description

TaskFieldRenderer has a className() method acting like a data getter (get a component's class). The data is used later to add to an HTMLElement. This PR replaces it with a addClassName() method that add the class to the target HTMLElement.

Previously className() was written in mind with usage in tests as well (As a provider of data). However #2545 removed the relation between the data in TaskFieldRenderer and the data in the tests.

Motivation and Context

• have testable behaviour instead of data in TaskFieldRendrer
• hide implementation details of adding classes inside TaskFieldRenderer
• have uniform behaviour in TaskFieldRenderer: addClassName(), addDataAttribute(), ...

How has this been tested?

New unit tests, breaking the code.

Types of changes

Internal changes:

• Refactor (prefix: refactor - non-breaking change which only improves the design or structure of existing code, and making no changes to its external behaviour)
• Tests (prefix: test - additions and improvements to unit tests and the smoke tests)

Checklist

• My code follows the code style of this project and passes yarn run lint.
• My change has adequate Unit Test coverage.

Terms

• My contribution follow this project's contributing guide
• I agree to follow this project's Code of Conduct

[claremacrae]

Nice - thank you.

Sorry for the inconvenience, but please could you rebase when you have time,

[ilandikov]

Sure, done =)