chore: explicitly declare ext-tokenizer as PHP extension

[drupol]

This project relies on the tokenizer PHP extension, but it is not explicitly declared in composer.json. It is assumed to be available in the PHP distribution, which is not always the case. This package makes use of several T_* constants, which require the tokenizer extension. Without it, the following error may occur:

Fatal error: Uncaught Error: Undefined constant "Microsoft\PhpParser\T_CLASS_C" in ...

This PR adds the missing ext-tokenizer requirement to the require section.

[drupol]

No feedback since a couple of month, closing this.