PHP8: Adds support for throw expressions

[cfroystad]

RFC

Changes the throw statement into a throw expression in accordance with PHP 8. The change has been tested with all edge cases kindfully listed by the RFC author.

According to the RFC and the test suite, the change does not break backwards compatability.

Notable changes:

• Renamed throw_statement to throw_expression
• Changed throw_expression behaviour to mach expression behaviour

Checklist

• All tests pass in CI.
• There are sufficient tests for the new fix/feature (see tests in expressions.txt)
• Grammar rules have not been renamed unless absolutely necessary (1 rules renamed)
• The conflicts section hasn't grown too much (0 new conflicts)
• The parser size hasn't grown too much (master: 1664 , PR: 1666)

[maxbrunsfeld]

Very nice work ⚡