[QA] Add PHP-CS-Fixer

[veewee]

Automatically check some CS rules as described in #30

FYI : the config and the fixes are in 2 separate commits. I can change those independently if you wish to change a setting.

[veewee]

The latest psalm version gives this issue:

ERROR: RedundantCondition - src/Psl/Str/convert_encoding.php:27:5 - Type string for $result is never false (see https://psalm.dev/122)
    Psl\invariant(false !== $result, 'Unable to convert $string from $from_encoding to $to_encoding.');


ERROR: RedundantCondition - src/Psl/Str/convert_encoding.php:27:19 - string can never contain false (see https://psalm.dev/122)
    Psl\invariant(false !== $result, 'Unable to convert $string from $from_encoding to $to_encoding.');

Haven't touched the file. Is it safe to remove the invariant check? Or shall I add an ignore for that rule?
Looks like it always returns a string: https://www.php.net/manual/en/function.mb-convert-encoding.php

[coveralls]

Pull Request Test Coverage Report for Build 645

• 24 of 24 (100.0%) changed or added relevant lines in 14 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 642:	0.0%
Covered Lines:	1748
Relevant Lines:	1748

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💛 - Coveralls

[azjezz]

okay; you can go ahead and remove that check.