feat(analysis/fourier): convergence of Fourier series (#17913)
This PR adds a straightforward but useful criterion for convergence of Fourier series: for a continuous periodic function `f`, if the sequence of Fourier coefficients of `f` is absolutely summable, then the Fourier series converges uniformly, and hence pointwise everywhere, to `f`. (Note that it is obvious in this case that the Fourier series converges uniformly to _something_, the difficult part is that the limit is actually `f`.)
Co-authored-by: Junyan Xu <junyanxu.math@gmail.com>
