feat(analysis/fourier): Poisson summation (#18392)
This PR proves Poisson's summation formula for continuous functions on the real line satisfying the following (somewhat inexplicit) hypothesis: for each compact $K$, the sum $\sum_{n \in \mathbb{Z}} \sup_{x \in K} \|f(x + n)\|$ converges. (In a future PR it will be shown that this is automatically satisfied when f has exponential decay, giving a less general but rather easier-to-apply result.)
