Php-parser: Support 7.3

[ichiriac]

• [x] https://wiki.php.net/rfc/list_reference_assignment
• [x] https://wiki.php.net/rfc/trailing-comma-function-calls
• [x] https://wiki.php.net/rfc/flexible_heredoc_nowdoc_syntaxes

[ichiriac]

related to https://github.com/prettier/plugin-php/issues/854

[alexander-akait]

/cc @ichiriac better implement flexible_heredoc_nowdoc_syntaxes instead https://github.com/glayzzle/php-parser/issues/205

I gradually fix ast, no need rush, i will send a PR

[ichiriac]

The flexible_heredoc_nowdoc_syntaxes will be hard to implement, it's mainly on the lexer - I'll start when I'll have few hours to work on it

[alexander-akait]

/cc @ichiriac can you help with flexible_heredoc_nowdoc_syntaxes? php 7.4 doesn't have many problems with implementation, it is only one difficult thing

[ichiriac]

It's implemented on local copy, just need to test edge cases :smile:

[alexander-akait]

@ichiriac feel free to send a PR, will be great look on this

[ichiriac]

merged on https://github.com/glayzzle/php-parser/pull/444