Make WordDelimiterGraphFilter a Tokenizer [LUCENE-8516]

Being able to split tokens up at arbitrary points in a filter chain, in effect adding a second round of tokenization, can cause any number of problems when trying to keep tokenstreams to contract. The most common offender here is the WordDelimiterGraphFilter, which can produce broken offsets in a wide range of situations.

We should make WDGF a Tokenizer in its own right, which should preserve all the functionality we need, but make reasoning about the resulting tokenstream much simpler.

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Migrated from [LUCENE-8516](https://issues.apache.org/jira/browse/LUCENE-8516) by Alan Woodward (@romseygeek), updated Oct 04 2018
Attachments: [LUCENE-8516.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-8516/LUCENE-8516.patch)