Add support for text shaping

[RagibHasin]

As I understood the code, printpdf does no text shaping and generates a glyph ID for each code point.

In #73 @fschutt mentioned that, ideally, the shaping engine should be pluggable. Recently, yeslogic/allsorts has emerged as a viable option for text shaping and font substitution in pure Rust, which is already being used in production.

Is that plan still valid? If so, would it be acceptable to switch font subsetting to allsorts along with text shaping?