Declare a public field in JapaneseEra for the era starting May 2019

Summary

Declare a public field in java.time.chrono.JapaneseEra for the new era which starts May 1st, 2019.

Problem

Without a field, developers have to call JapaneseEra.of(int) with the value 3 in order to obtain an instance of JapaneseEra that represents the new era; this is not intuitive.

Solution

Declare a public static final field that represents the new era, mirroring the pre-existing fields such as JapaneseEra.HEISEI.

Specification

Add a new era field in java.time.chrono.JapaneseEra:

/**
 * The singleton instance for the 'Reiwa' era (2019-05-01 - )
 * which has the value 3. The end date of this era is not specified, unless
 * the Japanese Government defines it.
 */
public static final JapaneseEra REIWA;

Change the specification of the method java.time.chrono.JapaneseChronology#eraOf(int) from:

/**
 * Returns the calendar system era object from the given numeric value.
 *
 * See the description of each Era for the numeric values of:
 * {@link JapaneseEra#HEISEI},{@link JapaneseEra#SHOWA},{@link JapaneseEra#TAISHO},
 * {@link JapaneseEra#MEIJI}), only Meiji and later eras are supported.
 * ...

to:

/**
 * Returns the calendar system era object from the given numeric value.
 * The numeric values supported by this method are the same as the
 * numeric values supported by {@link JapaneseEra#of(int)}.
 * ...

Also, add the following row in "japanese"/Calendar.ERA section of java.util.spi.CalendarNameProvider's chart in its class description:

<tr>
  <th scope="row" style="font-weight:normal">5</th>
  <td >Reiwa</td>
</tr>   

Link to the specdiff: http://cr.openjdk.java.net/~naoto/8174268/specdiff.00/overview-summary.html