PHP 8.0 | Tokenizer/PHP: add support for nullsafe object operator

[jrfnl]

This PR addresses the tokenizer changes needed for #3040. Once this PR has been merged, the sniff specific changes will be pulled with a PR per sniff.

PHP 8 introduces a new object chaining operator ?-> which short-circuits moving to the next expression if the left-hand side evaluates to null.

This operator can not be used in write-context, but that is not the concern of the PHPCS Tokenizers\PHP class.

This commit:

• Defines the token constant for PHP < 8.0.
• Adds a backfill for the nullsafe object operator for PHP < 8.0 to the PHP tokenizer.
• Adds the token to applicable token lists in the PHP and base tokenizer class, like the one used in the short array re-tokenization.
• Adds perfunctory unit tests for the nullsafe object operator backfill.
• Adds a unit test using the operator to the tokenizer tests for the short array re-tokenization.

Refs:

• https://wiki.php.net/rfc/nullsafe_operator
• php/php-src@9bf1198

[jrfnl]

@gsherwood Two FYIs:

1. The nightly build tests failure is not due to this PR, but to PHP 8 changes related to named identifiers as one token - see PHP 8.0 | Proposal for handling "namespaced names as single token" #3041
2. While this PR does introduce a new token, that token is a backfill for a PHP native token, not a PHPCS specific one, so with that in mind, I believe this PR can be milestoned for 3.5.6.

[gsherwood]

Thanks. I think I've now unblocked those other PRs you have.

[jrfnl]

@gsherwood One set of them for sure, though I also have sets ready for the union types and constructor property promotion changes.
I'll send in the ones which were blocked by this PR in a moment.