Fix multiline text not preserved when using "maxWidth" property

[listopadiya]

There was a problem when using multiline text and maxWidth text property simultameously, maxWidth was kind of rewriting existing text line splitting. It could be seen with multiline text inside table cell (#2891) or just with ordinary text like this:

doc.setFontSize(16);
doc.text("This is first line.\nThis is next line.", 20, 20, {"maxWidth": 25});
doc.text(["This is first line.","This is next line."], 20, 60, {"maxWidth": 25});

With this PR lines of text defined by user are preserved when putting width restriction on them. It works for both multiline-as-an-array and multiline-as-formatted-with-linebreak-symbols texts. Also cell line height calculation was fixed, so that it works for both variants described above.

There is String Splitting example in live demo, it works correctly, but looks like it uses splitTextToSize method directly.

Basic checks done:

• different combinations checked manually in Live Demo
• two problem-specific unit tests are added
• npm run test-local pass except one test (which is not passing in master branch for me as well, so probably it is my local browser-related problem or whatever)