Ability to detect --update flag during testing of snapshots

Clear and concise description of the problem

I am using .toMatchFileSnapshot() (the single snapshot file was getting out of hand), however, I would like to be able to make sure any unused snapshots are getting cleaned up. (e.g. if a previous test using a snapshot was deleted / has had its name changed, this will result in a file in source control needing to be cleaned up since there's a one to one mapping between these snapshot files and tests)

Suggested solution

Add an updatingSnapshots boolean to the test context.

Alternative

Pass in an additional environment variable

$ UPDATE_SNAPSHOTS=true npx vitest run 'snapshots.test' --update

...
afterAll(() => {
  // ⬇️ It would be better if this came from the context - at the moment, as far as
  // I can see, there's no way to detect if the --update flag was used
  const updateSnapshots = Boolean(process.env.UPDATE_SNAPSHOTS);

  const unusedSnapshotFileNames = fs
    .readdirSync(resolve(SNAPSHOTS_DIRECTORY_NAME))
    .filter((file) => usedSnapshotFileNames.includes(file) === false);

  if (updateSnapshots) {
    for (const file of unusedSnapshotFileNames) {
      fs.unlinkSync(resolve(SNAPSHOTS_DIRECTORY_NAME, file));
    }
    console.info(
      `🚮 ${unusedSnapshotFileNames.length} unused snapshots deleted`,
    );
  } else {
    throw new Error(
      `Unused snapshots found: ${unusedSnapshotFileNames.join(', ')}`,
    );
  }
});
...

Additional context

No response

Validations

• Follow our Code of Conduct
• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.

Start with the test-context documentation and the .toMatchFileSnapshot() behavior, then trace how vitest run ... --update is represented during a test. Use the snapshot test command from the issue to verify that the test context exposes whether snapshot updating is enabled, and confirm the value differs between normal and --update runs.