Add the space-discarding feature

[isuffix]

This adds the space-discarding feature as described at typst/typst#7350 (comment), although I have changed from "whether a writing system uses spaces between words" to "whether a writing system uses spaces at all."

I will leave this PR description short as the code itself contains a plenty of discussion of rationale and implementation considerations, along with my research into the usage of space characters in various writing systems.

There is a lot of writing here, so I would really appreciate help with checking for typos and inconsistencies, as it has become hard for me to consider everything with fresh eyes. I am very amenable to suggestions :)

I will also restate that I only speak English and while I have tried to do good research, I am not infallible. I would appreciate any input from native speakers of Chinese or Japanese or any of the other writing systems discussed in the PR.

I would also like to thank @r12a for his wonderfully detailed orthography descriptions and script comparison table, without which this PR would not be nearly as complete or authoritative. If you're reading this, I would love any feedback you could provide.

[r12a]

Tibetan comes to mind as an example of an orthography that doesn't use (ASCII) spaces but does have delimiters (syllable-based) which could break a line in the source code, but which should not incur an extra space when stitching things together (see Tibetan Orthography Notes). hth