New meaning of equal sign in StandardQueryParser [LUCENE-5664]

The StandardSyntaxParser.jj has (undocumented?) support for the <, <=, > and => operators that generate a TermRangeQueryNode. The equal operator, however, behaves just like the colon and produces a regular Term node instead of a TermRangeQueryNode.

I've been using the attached patch in a project where we had to be able to query the exact value of a field and I'm hoping there is interest to apply it upstream.

(Note that the colon operator works just as before, producing TermQuery or PhraseQuery nodes.)

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Migrated from [LUCENE-5664](https://issues.apache.org/jira/browse/LUCENE-5664) by Martin Blom, updated Oct 10 2018
Attachments: [LUCENE-5664.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-5664/LUCENE-5664.patch)

Start with StandardSyntaxParser.jj and compare the existing handling of <, <=, >, and => operators with the attached LUCENE-5664.patch. Verify that the equal operator produces a TermRangeQueryNode for exact-value queries while the colon operator continues producing Term or PhraseQuery nodes.

Written by the indexing model from the issue text.