Add `wk` unit for weeks

[jkrumbiegel]

While certainly not the most used time unit, we have a need for wk in our project because Unitful also defines this by default and we want to ascertain compatibility. I hope you don't mind adding this one?

[jkrumbiegel]

Ok by chance I did this right at the same time as https://github.com/SymbolicML/DynamicQuantities.jl/pull/81 so I'm going to close this one again

[MilesCranmer]

Quite a coincidence to have PRs within 9 minutes of eachother adding the same thing! 🤯

[github-actions[bot]]

Benchmark Results

	main	db8141862a22e2...	t[main]/t[db8141862a22e2...]
Quantity/creation/Quantity(x)	3.1 ± 0.2 ns	3 ± 0.2 ns	1.03
Quantity/creation/Quantity(x, length=y)	3.9 ± 0.4 ns	4 ± 0.4 ns	0.975
Quantity/with_numbers/*real	3.6 ± 0.2 ns	3.8 ± 0.2 ns	0.947
Quantity/with_numbers/^int	13.1 ± 3.3 ns	12.1 ± 3 ns	1.08
Quantity/with_numbers/^int * real	12.1 ± 4.1 ns	12.4 ± 4 ns	0.976
Quantity/with_quantity/+y	5.9 ± 0.2 ns	6.1 ± 0.4 ns	0.967
Quantity/with_quantity//y	3.8 ± 0.1 ns	4.1 ± 0.3 ns	0.927
Quantity/with_self/dimension	1.9 ± 0.1 ns	1.7 ± 0.1 ns	1.12
Quantity/with_self/inv	4.5 ± 0.2 ns	3.6 ± 0.3 ns	1.25
Quantity/with_self/ustrip	1.7 ± 0.1 ns	1.6 ± 0.1 ns	1.06
QuantityArray/broadcasting/multi_array_of_quantities	0.363 ± 0.19 ms	0.347 ± 0.15 ms	1.05
QuantityArray/broadcasting/multi_normal_array	0.102 ± 0.023 ms	0.103 ± 0.026 ms	0.987
QuantityArray/broadcasting/multi_quantity_array	0.349 ± 0.033 ms	0.353 ± 0.036 ms	0.988
QuantityArray/broadcasting/x^2_array_of_quantities	0.0745 ± 0.02 ms	0.0724 ± 0.025 ms	1.03
QuantityArray/broadcasting/x^2_normal_array	12.6 ± 1.3 μs	12.5 ± 1.2 μs	1.01
QuantityArray/broadcasting/x^2_quantity_array	13.2 ± 1.4 μs	13.3 ± 1.8 μs	0.992
QuantityArray/broadcasting/x^4_array_of_quantities	0.19 ± 0.044 ms	0.186 ± 0.051 ms	1.02
QuantityArray/broadcasting/x^4_normal_array	0.0935 ± 0.017 ms	0.0938 ± 0.019 ms	0.997
QuantityArray/broadcasting/x^4_quantity_array	0.115 ± 0.021 ms	0.115 ± 0.025 ms	0.996
time_to_load	0.263 ± 0.016 s	0.247 ± 0.0073 s	1.06

Benchmark Plots

A plot of the benchmark results have been uploaded as an artifact to the workflow run for this PR. Go to "Actions"->"Benchmark a pull request"->[the most recent run]->"Artifacts" (at the bottom).