Add PHP 8.4 support

[asgrim]

As it says :)

[asgrim]

Currently blocked on this by upstream packages:

• azjezz/psl
• vimeo/psalm

$ php84composer up --with-all-dependencies
Loading composer repositories with package information
Updating dependencies
Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - Root composer.json requires azjezz/psl ^2.9 -> satisfiable by azjezz/psl[2.9.0, 2.9.1].
    - azjezz/psl[2.9.0, ..., 2.9.1] require php ~8.1.0 || ~8.2.0 || ~8.3.0 -> your php version (8.4.0beta4) does not satisfy that requirement.
  Problem 2
    - vimeo/psalm[5.22.0, ..., 5.25.0] require php ^7.4 || ~8.0.0 || ~8.1.0 || ~8.2.0 || ~8.3.0 -> your php version (8.4.0beta4) does not satisfy that requirement.
    - Root composer.json requires vimeo/psalm ^5.22 -> satisfiable by vimeo/psalm[5.22.0, ..., 5.25.0].

[davisenra]

Looks like vimeo/psalm is under some effort to reach PHP 8.4.

vimeo/psalm#10928