Allow multi-line bold, italics, and strikethrough

[ChrisPenner]

Overview

fixes #4669

The formatter will break up bold, italic, and strikethrough sections over multiple lines, but the lexer doesn't parse multi-line sections like that.

I changed it so the parser can handle multiline bold, italics and strikethroughs as long as they're part of the same paragraph.

Implementation notes

• Adds whitespaceWithoutParagraphBreak helper which parses a block of whitespace with at most 1 newline.
• Use the new whitespace helper when parsing more docs within a bold, italic, or strikethrough.

Test coverage

• Added regression test to doc formatter