feat: Add reporter for vitest-evals

[Luca Forstner (lforst)]

Adds a reporter for https://www.npmjs.com/package/vitest-evals

Setup:

// vitest.config.ts
import BraintrustVitestEvalsReporter from "braintrust/vitest-evals-reporter";
import { defineConfig } from "vitest/config";

export default defineConfig({
  test: {
    reporters: [
      "vitest-evals/reporter",
      new BraintrustVitestEvalsReporter({
        projectName: "my-project",
      }),
    ],
  },
});

or

reporters: [
  "vitest-evals/reporter",
  ["braintrust/vitest-evals-reporter", { projectName: "my-project" }],
]

We are emitting these scorers:

{
  pass: 1 | 0,
  avg_score?: number | null,
  [judgeName]: number | null
}

[Abhijeet Prasad (AbhiPrasad)]

should we prefix with vitest? To reduce the chance of user collision if they decide to call their score avg_score or pass?