add support for Laravel 8

[tabacitu]

Good news @barryvdh ! it looks like all we need to do, to provide Laravel 8 support, is to allow the installation 🥳 I have tested the most important features, and they work just fine:

---

If anybody else wants to test this, or upgrade to Laravel 8 before this PR gets merged (or an alternative one), please:

• add my fork to your composer.json's repositories section:

    "repositories": [
        {
            "type": "vcs",
            "url": "https://github.com/tabacitu/laravel-elfinder"
        }
    ],

• run composer require barryvdh/laravel-elfinder:"dev-l8-compatibility as 0.4.99"

Note that I have no intention of keeping that fork. Immediately after the official repo provides Laravel 8 support, I'll delete it. At that point you should undo the changes above, run composer update and it'll work just fine.

[nguyentranchung]

php version should >= 7.3

[tabacitu]

Indeed, Laravel 8 does require PHP 7.3+, a requirement which should be imposed just fine thanks to the fact that illuminate/support is a dependency and has it - https://github.com/illuminate/support/blob/master/composer.json

This PR intentionally does not force a PHP version bump, it adds L8 support as a non-breaking change.

If someone installs this package on

• Laravel 7, then the illuminate/support version will require php 7.2.5+
• Laravel 8, then the illuminate/support version will require php 7.3+

No need to bump the required PHP version here too, in my opinion.

[barryvdh]

Thanks!