refactor(reporters): base reporter readability improvements

[AriPerkkio]

Description

• Improves readability of base reporter's code
• While building other reporter related feature, I had to study base reporter throughly. The code wasn't easy to read so I did some improvements while looking into it. Let's merge these improvements here as their own PR in order to reduce changes of the actual up-coming feature PR.

"Readability" is opinionated term so feel free to suggest changes if some lines don't make sense.

Please don't delete this checklist! Before submitting the PR, please make sure you do the following:

• It's really useful if your PR references an issue where it is discussed ahead of time. If the feature is substantial or introduces breaking changes without a discussion, PR might be closed.
• Ideally, include a test that fails without this PR but passes with it.
• Please, don't make changes to pnpm-lock.yaml unless you introduce a new test example.

Tests

• Run the tests with pnpm test:ci.

Documentation

• If you introduce new functionality, document it. You can run documentation with pnpm run docs command.

Changesets

• Changes in changelog are generated from PR name. Please, make sure that it explains your changes in an understandable manner. Please, prefix changeset messages with feat:, fix:, perf:, docs:, or chore:.

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[hi-ogawa]

👏