fix(config): preserve other deploy fields when writing org/app

[velikodniy]

Summary

Problem

deno deploy updates the org and app fields in the config. However, it doesn't preserve other fields in the deploy block.

For example, after running deno deploy this config:

 "deploy": {
    "org": "my-org",
    "app": "my-app",
    "exclude": [
      "!dist"
    ]
  }

becomes just

  "deploy": {
    "org": "vlcdn",
    "app": "parkruns"
  }

Changes

The solution is to update the org and app fields directly rather than replacing the entire deploy block.

Tests

Since writeConfig is not exported, the new behaviour is tested via executing the deploy handler.

[velikodniy]

The tests failed, but it's because the DENO_DEPLOY_TOKEN isn't set. This PR is a fork PR, so the secret is empty.

Note that it crashes in the tests added before this PR.