feat: Nested Workspace Summary Categories

[aswa2ds]

enabled nested workspace summary categories. Categories with dots can be split to nested categories. And it's allowed to show the category title and file link in the same level category, like school.science --a file link-- and school.test --a category-- in the demo below . #973

And I found that some users wanna ignore some files in workspace summary. So do I. #953
Actually we can use the current ability to realize it. While the categories field was filled with empty list like: []. Like my index.norg file in the demo above. Because of the judge condition below. It hits no branch, and be ignored in the loop below the if-else. And I think it's more flexible than just configuring.

[vhyrro]

Generally the code looks good to me! Just one thing - it might make sense to precompute the value of lib.title(path) once in a variable (e.g. local path_as_title = lib.title(path)) and use it throughout the code instead of invoking it several times every time we need it. The lib.title() function is a bit expensive to run so it would be best if we could run it as few times as possible. If you could fix that up that would be great!

[aswa2ds]

😞Had some stupid behaviors. It has been changed as you say. Sorry for ignoring the cost of repeat method call.

[vhyrro]

Huh, github seems to be messing things up. Didn't mean to close this, give me a moment :p

[vhyrro]

Okay it seems like I messed something up. I merged the changes manually with the main branch because github apparently decided to autoclose this PR. I gave attribution in the commit message. Thank you for the pull request! 💜