SpanishLightStemmer stemming errors [LUCENE-7420]

SpanishLightStemmer only applies stemming if the original word is 5 characters or longer. It tries to removing gender and numbers, removing final vowel and s if they exist.

So, perro, perra, perros and perras (dog, bitch, dogs and bitches) all become 'perr'.
The problem arises with shorter words. Gatos and Gatas (cats / female cats) all become 'gat', but the singular forms (gato, gata) are below the 5 character threshold, so they wouldn't be stemmed.

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Migrated from [LUCENE-7420](https://issues.apache.org/jira/browse/LUCENE-7420) by J Pardos, updated Mar 07 2018
Attachments: [SpanishLightStemmer.java.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-7420/SpanishLightStemmer.java.patch)

Locate the SpanishLightStemmer implementation and review the attached SpanishLightStemmer.java.patch first. Check the length threshold and existing gender, number, and final-vowel handling using the gato/gata and gatos/gatas examples. Done means singular and plural forms stem consistently without regressing the perro examples.

Written by the indexing model from the issue text.