Remove PHPCS and replace with ECS

[schlessera]

This PR removes PHPCS and replaces it with ECS - Easy Coding Standard.

This has the following advantages:

• Use of both PHPCS and PHPCSF via one PHP config file
• Advanced caching for quick rechecking during development
• Possibility to check code snippets in markdown files

[schlessera]

Overall, ECS seems to offer advantages over PHPCS for this repo. It is only supported right now from PHP 7.1 onwards, though.

So, if we want to go forward with this, we'll have to do one of:

1. manually remove the ECS dependency on PHP < 7.1
2. download the ECS Phar on-demand for code style checks
3. bump PHP minimum version from PHP 5.6 to PHP 7.1

[schlessera]

Something to note: the original idea of trying to move to ECS is that it will allow the code generation for the validator spec to be run through the ECS fixed, which reliably enforces the exact same code style than for the rest of the code. This is unfortunately not easily done with PHPCS, as its fixing only covers a part of its ruleset.

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