Remove pkg/errors dependency

[AndrewRPorter]

Summary

• replace first-party github.com/pkg/errors usage with standard-library errors and fmt.Errorf wrapping
• update error-chain tests and generated Twirp compatibility comments to use errors.Is/errors.As
• remove the dependency from dep manifests and delete its vendored package

Testing

• go test github.com/twitchtv/twirp github.com/twitchtv/twirp/internal/descriptors github.com/twitchtv/twirp/internal/gen/typemap github.com/twitchtv/twirp/protoc-gen-twirp github.com/twitchtv/twirp/clientcompat
• go test -vet=off github.com/twitchtv/twirp/internal/twirptest

internal/twirptest uses -vet=off because the current Go toolchain reports pre-existing formatting diagnostics in unrelated test lines.