Support nanosecond precision time coordinates

[maxrjones]

pydata/xarray#8575 added an overflow warning for time coordinates in Xarray:

OverflowError: Not possible to cast encoded times from dtype('int64') to dtype('int32') without overflow. Consider removing the dtype encoding, at which point xarray will make an appropriate choice, or explicitly switching to a larger integer dtype.

which users may encounter due to our standardization of time coordinates as int32 (xref #87 and #117). It may be worth adding int64 support in zarr-js and allowing overriding the time encoding in ndpyramid to support nanosecond precision times.