Fix makie 0.24

[rossviljoen]

This PR does two things for the MakieExt:

1. Fixes the issue raised here: Update MakieExt compat #15 (comment)
2. Defaults the custom_epoch used for the conversion to UNIX_EPOCH. This is because the current behaviour (picking the custom epoch to be the last plotted point in the axis) causes multiple axes in the same figure to be misaligned when they have different epochs. It is still possible to set a custom epoch for higher precision on recent dates with:

fap = Makie.lines(times, data)

fap.axis.dim1_conversion[].custom_epoch[] = unix_now()
reset_limits!(fap.axis)

(Unfortunately, constructing the UnixTimeConversion directly to pass to the axis requires the rather ugly Base.get_extension(UnixTimes, :UnixTimesMakieExt).UnixTimeConversion, as it isn't exported by UnixTimes.)

Tested on Makie@{0.22, 0.23, 0.24}

@JoaoAparicio

[JoaoAparicio]

#17