Add merge_aligned_spans

[ericphanson]

This would be https://github.com/beacon-biosignals/TimeSpans.jl/blob/9f80d852a51b6df3d263b848c51c306c0d43fa40/src/TimeSpans.jl#L311-L365C72 but with a new shortest_aligned_span_containing instead of shortest_timespan_containing, and with checks that all spans have the same sample rate, and using by=x -> x.first_index instead of by=start where applicable.

If indeed all spans have the same sample rate, this should be the same as

merge_aligned_spans(f, aligned_spans) = AlignedSpan.(sample_rate, merge_spans(f, aligned_spans))

except more efficient and such that f gets aligned spans rather than timespans (or a mix of aligned spans and timespans).

Alternatively, we could try to update TimeSpans.jl to allow shortest_timespan_containing to be configurable in merge_spans, and could pass in our own shortest_aligned_span_containing instead. (And just pay the time for computing times with start rather than first_index, but that's probably fine).