Add new option foldChars

[eemeli]

Context: eemeli/prettier-plugin-properties#15, CC @murdos

Makes the preferred folding point customizable. By default, lines that need folding will add an escaped newline after the last possible \f, \t, space, or . on the line. To pick a different (or empty) character set, set the foldChars option with a string of such characters.