feat: merge simplePrefixing allocator ignore with other file ignores

[Leptopoda]

Using Allocator.simplePrefixing() with a library decalaration resulted in bad formatted code:

Library((b) => b
  ..docs.add(
    '/// My favorite library.',
  )
  ..body.add(Field((b) => b
    ..name = 'test'
    ..modifier = FieldModifier.final$
    ..assignment = Code.scope((a) => '${a($LinkedHashMap)}()'))))

was generating:

/// My favorite library.
library; // ignore_for_file: no_leading_underscores_for_library_prefixes

import 'dart:collection' as _i1;

final test = _i1.LinkedHashMap();

This PR merges the no_leading_underscores_for_library_prefixes ignore with the other file level ignores for better readable code.

---

• [x] I’ve reviewed the contributor guide and applied the relevant portions to this PR.

Contribution guidelines:

• See our contributor guide for general expectations for PRs.
• Larger or significant changes should be discussed in an issue before creating a PR.
• Contributions to our repos should follow the Dart style guide and use dart format.
• Most changes should add an entry to the changelog and may need to rev the pubspec package version.
• Changes to packages require corresponding tests.

Note that many Dart repos have a weekly cadence for reviewing PRs - please allow for some latency before initial review feedback.