feat: Introduce multiline_string_to_heredoc fixer

[mvorisek]

fix #7554

Multiline strings breaks indentation visually, this fixer fix such strings to heredoc/nowdoc and with heredoc_indentation the visual code flow is then much more better.

[mvorisek]

PR is done. Tested on the whole repo, all unescaping should be done correctly.

[Wirone]

@mvorisek I believe this fixer should be run after no_useless_concat_operator, so multiple, concatenated multi-line strings are not transformed into multiple heredocs, instead of one merged. Integration test for that should be added.

[mvorisek]

I believe this fixer should be run after no_useless_concat_operator, so multiple, concatenated multi-line strings are not transformed into multiple heredocs, instead of one merged. Integration test for that should be added.

This is not possible as this fixer must be run before escape_implicit_backslashes which has higher priority.

no_useless_concat_operator should however handle heredoc contatenations, so in the end, it should be runnable before/after - #7662

[Wirone]

@mvorisek I know you want to help by keeping PRs up-to-date, but rebasing all of them does not make sense because after merging any of them the rest is outdated and they just waste CI capacity 😅. I do rebase after the review, so when it's approved and CI passes it gets merged. Parallel workflows take more time because some of the jobs are waiting for available runners. Let's do one after another 🙂.

[Wirone]

Minor improvements 🙂.

[Wirone]

I don't know if I can foresee possible problems that could emerge with this particular implementation, but looks good enough as initial version, so I'm fine with it - we can improve/fix it later if users report something 🙂.

[Wirone]

Thank you @mvorisek 🍻!