Benchmark by group

[BeniCheni]

Resolves #26425

• Implement the byGroup benchmark config option
• Add an example with buffer-from-by-group.js benchmark
• Update writing-and-running-benchmarks.md to mention the byGroup option:

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• documentation is changed or added
• commit message follows commit guidelines

example in "buffer-from-by-group.js" benchmark

const bench = common.createBenchmark(main, {
  groupA: {
    source: [
      'array',
      'arraybuffer',
      'arraybuffer-middle',
      'buffer',
      'uint8array',
      'string',
      'string-utf8',
      'string-base64',
      'object',
    ],
    len: [10, 2048],
    n: [2048]
  },
  groupB: {
    source: [
      'buffer',
      'string',
    ],
    len: [2048],
    n: [2048]
  }
}, { byGroup: true });

[BeniCheni]

Closing this draft PR from a test. Sorry about the tiny noise.