dev: simplify cutVal implementation

[alexandear]

The PR refactors the implementation of an unexported function cutVal in the printers package.

[ldez]

A character can use several runes so you cannot do that.
This is why utf8.DecodeRuneInString is used.

[alexandear]

A character can use several runes so you cannot do that. This is why utf8.DecodeRuneInString is used.

Does this mean that the TestTeamCity_cutVal is wrong? Because it is not failing on PR's changes.

https://github.com/golangci/golangci-lint/blob/v1.62.2/pkg/printers/teamcity_test.go#L95-L99

[ldez]

Does this mean that the TestTeamCity_cutVal is wrong?

No, it means that a test is validating only the cases defined inside the tests.

The current implementation of cutVal has some limitations too.

In fact, the current implementation and your proposal have the same limitations, so we can accept your PR.

For the details, emoji ZWJ sequences are not handled correctly, so a composite emoji can be cut improperly, but I think this is a minor edge case.