Add lineBreak: 'whitespace' to wrap lines only at spaces

[asmyshlyaev177]

What kind of change does this PR introduce?

A feature. It adds a new value for the existing lineBreak option.

pdfkit can break a line after a hyphen or a slash, not just at a space. That's correct by the Unicode line breaking rules, but it splits words and links that should stay whole. A tool that reads text back out of the PDF then gets them wrong. Résumé parsers, search, and copy and paste all do this.

Example 1: a hyphenated word

doc.text('a design-system', { width });

	Line 1	Line 2	A parser reads
Before	a design-	system	designsystem
After, with lineBreak: 'whitespace'	a	design-system	design-system

Example 2: a URL

doc.text('see https://github.com/foliojs/pdfkit', { width });

	Line 1	Line 2	A parser reads
Before	see https://github.com/foliojs/	pdfkit	a link to https://github.com/foliojs/, then the word pdfkit
After, with lineBreak: 'whitespace'	see	https://github.com/foliojs/pdfkit	the full link

There is no way around this in pdfkit today. The standard 14 fonts have no non-breaking hyphen (see #1040, where ‑ renders as blank space), and lineBreak: false means writing your own line wrapping.

What the option does

With lineBreak: 'whitespace', a line breaks only at:

• a space or other whitespace
• a newline
• a soft hyphen (­), which still shows a hyphen at the line end
• a zero-width space (​)

A word longer than the whole line is still split where the line ends, as it is today. The default behavior doesn't change. The option also works on a continued segment.

The option's type becomes boolean | 'whitespace'. If this PR is accepted, I'll update @types/pdfkit to match.

This PR doesn't fix #956, where the boundary between two continued segments is always a break point.

Checklist:

• Unit Tests: the default rules as they are today, both examples above, soft hyphen, zero-width space, newline, a word wider than the line, and a continued segment
• Documentation: docs/text.md
• Update CHANGELOG.md
• Ready to be merged