Change ordering of fields in typst.toml (discussion would be useful)

[SillyFreak]

The current typst.toml is missing some optional fields and IMO also orders them in a strange way (for example, there are four keys between keywords and categories, which are both about discoverability of packages via tag-like classifications). This PR changes the order to reflect what's described in typst/packages. It also links that page in the file for reference.

Although the order in the packages repo is a minimally opinionated default, I could also imagine some tweaks. Personally, I have the following comments:

• note that the first three are absolutely required and the first six are required for Universe, so that biases the ordering in the packages repo a bit: if you use the less-required fields, maybe you'd consider them more fitting at an earlier place.
• I'd put description earlier, somewhere between name and license
• I'd put compiler directly after entrypoint
• homepage is relatively rare, so I could imagine commenting it out by default
• I like exclude being last, it feels the least informative to a reader
• the [template] could be removed, but the example contains useful defaults so I think it's worth keeping
• [tool.mytool] could be removed or replaced by [tool.typst-test] - then it's a useful default and an example at the same time