v3: Don't count sequence indicator as part of indentation.

[monopole]

This PR removes 2 spaces from sequence indentations to account for space consumed by the sequence item indicator, and adds tests around the use of the SetIndent api.

This maintains sequence indentation consistency with gopkg.in/yaml.v2 (see the example at https://pkg.go.dev/gopkg.in/yaml.v2#readme-api-documentation) and conforms to the comment:

both the “-” indicator and the following spaces are considered
to be part of the indentation of the nested collection.

in the specification at https://yaml.org/spec/1.2/spec.html#id2772075

Kubernetes is currently pinned to gopkg.in/yaml.v2, and kustomize (now a part of kubectl) is pinned to v3. We'd like the marshalling of sequences by kustomize and kubectl (e.g. kubectl create deployment foo -o yaml --dry-run --image bar) to match. This means reverting the parts of ae27a74 and a5ece68 that dealt with sequence indentation.

There's more background in kubernetes-sigs/kustomize#3946. There's time pressure as we'd like kubectl and kustomize to be consistent before the next code kubernetes code freeze in early July, and this PR or a fork seem like the only options.

[monopole]

This indentation reversion should also help with moving kubernetes-sigs/yaml to v3, see kubernetes-sigs/yaml#18

[monopole]

Abandoning this for now; the change always resulted in a few bad failures of existing or new tests.