BREAKING: De-nest AttributeSource.AttributeFactory, #1145

[paulirwin]

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De-nest AttributeSource.AttributeFactory to be a top-level type.

Fixes #1145

Description

This PR de-nests AttributeFactory so it is a top-level type in Lucene.Net.Util. Because it is public, this is a breaking change.

This also changes AttributeSource.GetAttributeFactory() into the AttributeSource.AttributeFactory property, now that there is no longer a conflict with the nested type name.

[NightOwl888]

Thanks for the PR. Looks good.