Agree and implement CGL

[tomasnorre]

After briefly talking with @mk-mxp about coding guidelines #652 (comment) we agreed to split this out in a separate issue to follow up on this later.

We should agree on it first, though. I think tools like rector or other static code tools would be able to streamline this for us, when we have an agreement on what should be when.

[mk-mxp]

What I have collected so far, from old issues, the exercise code I have seen and my own favourites:

• PSR-12 as enforced by CI.
• No line length limit, but for readability try to keep it under
  • 80 chars for students code stubs.
  • 60 chars for test code.
  • Comments (especially DocBlocks) should wrap at 80 chars, too.
  • Keep in mind: in online editor, there is very limited choice for arranging code shown.
• Use snake_case for functions, camelCase for methods / properties and PascalCase for classes. The casing for names of variables in student facing code (mostly parameters) is not fixed - I'd prefer camelCase, too. And my wish for constants: ALL_UPPERCASE.
• Use classes, where the instructions would allow functions or methods (there are of course exercises using functions intentionally).
• Despite having discussed namespaces (in Namespaces and PSR-4 compliance #283), the way Exercism works strongly discourages namespaces / PSR-4 directory structures. I would use namespaces only in (yet not existent) exercises on namespaces and no PSR-4 directory structures at all (they make no sense here).
• Stick with ASCII for all names in code and use Unicode in strings sparsely. Provide comments on what type of Unicode codepoints you used (normalized to a composition schema, graphemes, ...) or encode the Unicode code points (\u{[0-9A-Fa-f]+}).

[tomasnorre]

I would personally go for camelCase for functions too, like getData() and not get_data.
I have used camelCase in all the PRs I have created recently.

I think it should be possible to have the phpcs scan for all this, in most IDEs. But would have to check when it differers in different files. But perhaps it is possible as we stick to conventions of e.g. example.php in the .meta folder.