feat(gqlerror): optimize Error method perfomance

[dienvt]

I found that for string concatenation, strings.Builder has better performance than bytes.Buffer. I have already tested the benchmark (cd gqlerror && go test -bench=.) for these approaches, and here are the results:

• When using bytes.Buffer

goos: darwin
goarch: arm64
pkg: github.com/vektah/gqlparser/v2/gqlerror
cpu: Apple M1 Max
BenchmarkError-10        4554457               262.9 ns/op

• When using strings.Builder

goos: darwin
goarch: arm64
pkg: github.com/vektah/gqlparser/v2/gqlerror
cpu: Apple M1 Max
BenchmarkError-10        5152414               218.0 ns/op

I have:

• Added tests covering the bug / feature
• [] Updated any relevant documentation

[coveralls]

coverage: 87.553%. remained the same
when pulling a421f45 on dienvt:gqlerror-optimize-performance
into 6361a74 on vektah:master.

[StevenACoffman]

Thanks!