FastVectorHighlighter - add position offset to FieldPhraseList.WeightedPhraseInfo.Toffs [LUCENE-3243]

Needed to return position offsets along with highlighted snippets when using FVH for highlighting.

Using the (#4214) patch I was able to get the fragInfo for a particular Phrase search. Currently the Toffs(Term offsets) class only stores the start and end offset.

To get the position offset, I added the position offset information in Toffs and FieldPhraseList class.

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Migrated from [LUCENE-3243](https://issues.apache.org/jira/browse/LUCENE-3243) by Jahangir Anwari, updated Jun 27 2011
Environment:
```
Lucene 3.2
```

Attachments: [CustomSolrHighlighter.java](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-3243/CustomSolrHighlighter.java), [LUCENE-3243.patch.diff](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-3243/LUCENE-3243.patch.diff)

Start by reading LUCENE-3243.patch.diff and the attached CustomSolrHighlighter.java, then trace FieldPhraseList and FieldPhraseList.WeightedPhraseInfo.Toffs in the current Lucene sources. The change is done when FVH phrase highlighting can return position offsets alongside each term's start and end offsets, with applicable tests passing.

Written by the indexing model from the issue text.