[jonasohrn]

Checklist:

• I've read and understood Contributing Guidelines.
• I've added the new resource at the end of its section.
• This resource is out there for a while, and actively maintained.
• This resource is popular enough and has at least a few hundred stars on GitHub.

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Adds a new Intervals and Periods section (right after Date, with a TOC entry) for libraries that do set operations on intervals and periods:

• periodz (https://github.com/jonasohrn/periodz) - Set operations (intersection, union, subtract) on periods of numbers, Dates and Day.js values.
• interval-operations (https://github.com/aanckar/interval-operations) - Dependency-free set operations on intervals and arrays of intervals.
• intervals-fn (https://github.com/js-republic/intervals-fn) - Functional, curried interval manipulation, including Allen's interval relations.
• ts-math-utils (https://jsr.io/@hrl/ts-math-utils) - Mathematical objects missing from JavaScript, such as intervals, interval sets and a range iterator.

Forgive me for bundling, and for the unticked boxes. I know the guidelines ask for one PR per suggestion. These four only make sense together under a new category, so I've put them in one PR so the section can be reviewed as a whole. Happy to split it if you prefer. None of them have hundreds of stars yet: interval set operations are a small niche, and I couldn't find a better-known library dedicated to them. Some are mature but quiet rather than actively developed, and periodz is new and my own library, in full disclosure. If the section doesn't meet the bar right now, I completely understand.