[Yaml] Do not treat quoted << keys as merge keys - #66621

In YAML, only a plain << key is a merge key. '<<' and "<<" are ordinary string keys. The parser treated quoted ones as merge keys too. As a result, the Dumper's own output can't always be read back, even though the Dumper already quotes the key:

$yaml = Yaml::dump(['<<' => 'foo']);   // "'<<': foo\n"
Yaml::parse($yaml);
// ParseException: YAML merge keys used with a scalar value instead of an array

Yaml::parse(Yaml::dump(['<<' => 'foo'], 0)); // "{ '<<': foo }"
// TypeError: Unsupported operand types: array + string

Yaml::parse("'<<': { a: 1 }");
// ['a' => 1] instead of ['<<' => ['a' => 1]]

The second example also shows that in inline mappings, a real merge key with a scalar value ({ <<: foo }), or a sequence of scalars ({ <<: [foo] }), crashed with a TypeError. The block parser already throws a ParseException in those cases, and now the inline parser does the same.

In Parser, the merge-key check now looks at the raw key text ('<<' === $values['key']). In Inline::parseMapping(), it uses the existing $isKeyQuoted. All the '<<' === $key checks go through the new $isMergeKey variable.

I found this by round-tripping random arrays through Yaml::dump() and Yaml::parse(). With the patch applied, a few hundred thousand cases on 6.4 came back identical. The new tests in InlineTest, ParserTest and DumperTest fail without the fix, and the Yaml test suite passes with it.

This also means that a quoted '<<' or "<<" key with a mapping value, which was merged before, is now kept as a key named <<, as YAML 1.1 and PyYAML do. So is !!str <<.

Merge-up to 8.2: in Parser.php, keep the $isMergeKey lines and drop the u modifier of #^&(?P<ref>[^ ]+)#, as 8.2 does. On 7.4, 8.1 and 8.2, DumperTest conflicts because both sides add a method at the end of the class: keep both.