Support PHP 7.4

[sirbrillig]

This is a meta issue to make sure we support PHP 7.4 (changes)

• Arrow functions: Add support for PHP 7.4 arrow functions #134
• Add CI build

[jrfnl]

Some additions to verify if these are supported/need support:

• If at any point the sniff looks at numbers: numeric literals with underscore separators
• Typed properties

FYI: PHPCSUtils has helper functions which already handle PHP 7.4 correctly for each of these + arrow functions.

[kkmuffme]

Just wanted to inquire if PHP7.4 is supported now?

[sirbrillig]

I think the main thing we need is support for arrow functions, which I started working on in #179 but I got kind of stuck. Handling implicit closure scope turns out to be complicated. Suggestions and assistance on that PR are welcome!

[sirbrillig]

Closed by #179