Adds support for explicit octal integer literal notation introduced in PHP 8.1.

rfc

This is a minor change, making it possible to explicitly use octal notation by using 0o or 0O ([digit: zero][letter: o case-insensitive])

This change is not relevant for the work on strings, due the the RFC clearly stating:

This RFC has no impact on the behaviour of numeric strings. “0o16” would still be interpreted as a string and only a string. Moreover, (int) "0o16" will continue to evaluate to 0.

Checklist:

• All tests pass in CI
• There are sufficient tests for the new fix/feature
• Grammar rules have not been renamed unless absolutely necessary (0 rules renamed)
• The conflicts section hasn't grown too much (0 new conflicts)
• The parser size hasn't grown too much (master: 2110, PR: 2110)