Normative: Don't add default formatting to lone era

Previously, if `era` was given by itself in the formatting options, the
default formatting for the other date/time components would be added.
This is inconsistent with what we do for `dayPeriod` where a lone
`dayPeriod` produces formats such as "in the afternoon".

The use case of obtaining the localized era name has also been mentioned
several times.

This change makes a lone `era` be passed to the format matcher algorithm
without added date/time components, and requires implementations to have
at least a year-month-day-era and year-month-era format available (which
looks to be widely available across languages in CLDR.)

See: #461

Author: ptomato

spec/datetimeformat.html

@@ -141,7 +141,7 @@ <h1>
         1. Else,
           1. Let _needDefaults_ be *true*.
           1. If _required_ is ~date~ or ~any~, then
-            1. For each property name _prop_ of « *"weekday"*, *"year"*, *"month"*, *"day"* », do
+            1. For each property name _prop_ of « *"weekday"*, *"era"*, *"year"*, *"month"*, *"day"* », do
               1. Let _value_ be _formatOptions_.[[&lt;_prop_&gt;]].
               1. If _value_ is not *undefined*, set _needDefaults_ to *false*.
           1. If _required_ is ~time~ or ~any~, then
@@ -261,6 +261,8 @@ <h1>Internal slots</h1>
             <li>weekday, year, month, day, hour, minute, second, fractionalSecondDigits</li>
             <li>weekday, year, month, day, hour, minute, second</li>
             <li>weekday, year, month, day</li>
+            <li>era, year, month, day</li>
+            <li>era, year, month</li>
             <li>year, month, day</li>
             <li>year, month</li>
             <li>month, day</li>