Add script to adjust letter spacing to avoid orphans

[jasongao97]

This PR adds a script to avoid orphans (too short last line) of paragraphs and list items. #666

For any multi-line text with a too short last line, the script adjusts the letter spacing incrementally until it either meets a desired minimum width or the letter spacing become too extreme.

For now, I set the minimum width to 40pt and the maximum letter spacing to 0.3pt.

Here is a test build showcasing different letter spacing compare to unadjusted text: letter-spacing-test.pdf

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[shiffman]

Hi @jasongao97 thank you for this! I'm going to leave this in a separate branch for now and build two PDFs. I'll run this by the publisher and check on the idea of adjusting letter spacing. Anything beyond 0.15pt looks a little extreme to me and in the end it may ultimately be better to just add a line break for "the" for examples like this.

[shiffman]

@jasongao97 would it be possible to apply this letter spacing adjustement to paragraphs that I specifically mark through a callout box. Then maybe I could try it out on some of the remaining orphans.

[shiffman]

@jasongao97 I think this is a useful and interesting reference for the future of the magic book project and documentation of this project even though we didn't end up incorporating it into the print PDF? What's the best way to archive this? Can we close this PR and add a reference to it somewhere else?