Remove some unused symbols

Add php-cs-fixer config and reformat

The formatting in this project has become something of a mess,
because it changed over time. Add a CS fixer config and reformat
to the desired st

Add php-cs-fixer config and reformat

The formatting in this project has become something of a mess,
because it changed over time. Add a CS fixer config and reformat
to the desired style, which is PSR-12, but with sane brace placement.

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Add visibility modifiers to constants

Closes GH-848.

Use PHP 8.0 token representation

Migrate everything to use PhpToken-compatible token representation,
rather than the legacy array/string representation.

Fix incorrect doc type

Add emulation for enum keyword

Make keyword emulation check case-insensitive

Handle flexible heredoc via TokenEmulator

Extend the interface to support preprocessing.

Add reverse emulation support

Add common KeywordEmulator

The logic for keyword emulation is always going to be the same.