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.

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[mosuem]

Closing as the dart-lang/code_builder repository is merged into the dart-lang/tools monorepo. Please re-open this PR there!