Deprecate Index features and introduce IndexEditor

[morozov]

This is a continuation of the rework started in #6685 (unique constraints) and #6728 (foreign key constraints), now applied to indexes. See the upgrade notes for details on the end-user impact.

Deprecation details

The Index#overrules() method is currently used by the ORM's SchemaTool (source). It should be easy to replace with a check if the the index columns are the same or a prefix of the PK columns. All other logic of overrules() (checking for the index type and the details of the partial index) is irrelevant in this case. I don't really understand how Index#overrules() is different from Index#isFulfilledBy() (besides the fact that it means the opposite).

Index#isFulfilledBy() will be available for now, but as soon as we get rid of implicit indexes (not very soon), it will become irrelevant to the DBAL. At that point, the ORM will be a more appropriate place for it.

[morozov]

This is not a breaking change. Index will catch this exception and turn into a deprecation notice. An InvalidState exception will be thrown only upon attempt to access the indexed columns via the new API:

dbal/tests/Schema/IndexTest.php

Lines 255 to 264 in feed861

	public function testNonPositiveColumnLength(): void
	{
	$this->expectDeprecationWithIdentifier('https://github.com/doctrine/dbal/pull/6787');
	$index = new Index('idx_user_name', ['name'], false, false, [], ['lengths' => [-1]]);
	$this->expectException(InvalidState::class);
	$index->getIndexedColumns();
	}