Spell checker puts a hard 3-letter limit on indexed words which affects suggestions [LUCENE-5707]

SVN r1055285 (committed for LUCENE-2391) introduced a hard limit on the minimum length required before including a word in the spell checker. This not only shows all 2-letter words as misspelled/not existing (unless the caller agrees that 2-letter words should not be checked for spelling) but 2-letter suggestions will never show up in a call to suggestSimilar(). It would be nice to have this limit reduced or, ideally, not hard-coded.


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Migrated from [LUCENE-5707](https://issues.apache.org/jira/browse/LUCENE-5707) by Scott Van Wart
Environment:
```
Windows 7 / IntelliJ IDEA
```

Attachments: [an.png](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-5707/an.png)

Start by locating the spell checker code that enforces the minimum indexed-word length, then reproduce the reported behavior for two-letter words and suggestions. Done means the hard-coded three-letter restriction is reduced or made configurable so two-letter words can be checked and returned by suggestSimilar().

Written by the indexing model from the issue text.