feat: @PhpCsFixer ruleset - enable multiline_string_to_heredoc

[mvorisek]

the fixer was introduced in #7665

the code is now much readable, especially test cases where multiline strings were used often

[Wirone]

+74,844 −58,592 😵. That's probably too big PR to handle.

[mvorisek]

+74,844 −58,592 😵. That's probably too big PR to handle.

@Wirone instead of landing #7709 first I decided to help with the review as much as possible, I changed no_trailing_whitespace_in_string fixer temporary to actually keep the trailing whitespaces that it would remove after multiline_string_to_heredoc applied normally.

The changes made in this PR should be solely CS. All strings should be effectively unchanged. Later, we can land improvements like in #7709, but such changes will change the strings, so let's not mix them in this large PR.

Can I please ask you to review this PR asap to prevent conflicts?

Here is how to review it:

1. checkout this PR branch
2. reset your local branch to the 1st commit
3. run fixer locally and verify if all the changes are equal to the 2nd commit (run fixer)
4. do 2nd/3rd step also for the next two run fixer commits