Fix behavior of complex

[MilesCranmer]

• complex(0.5u"km", 0.1u"km") -> quantity storing a complex number with length units
• complex(0.5, Quantity(0.1)) -> quantity storing a complex number, no units
• complex(0.5u"km", 0.1) -> DimensionError

[github-actions]

Benchmark Results

	main	fa076ea...	t[main]/t[fa076ea...]
Quantity/creation/Quantity(x)	3.1 ± 0.01 ns	3.1 ± 0.01 ns	1
Quantity/creation/Quantity(x, length=y)	3.11 ± 0.001 ns	3.11 ± 0.001 ns	1
Quantity/with_numbers/*real	3.1 ± 0.01 ns	3.11 ± 0.01 ns	0.997
Quantity/with_numbers/^int	8.36 ± 2.5 ns	8.36 ± 2.5 ns	1
Quantity/with_numbers/^int * real	8.37 ± 2.5 ns	8.38 ± 2.5 ns	0.999
Quantity/with_quantity/+y	5.27 ± 0.01 ns	5.27 ± 0.01 ns	1
Quantity/with_quantity//y	3.11 ± 0.001 ns	3.11 ± 0.001 ns	1
Quantity/with_self/dimension	1.56 ± 0.01 ns	1.55 ± 0.01 ns	1.01
Quantity/with_self/inv	3.11 ± 0.011 ns	3.11 ± 0.01 ns	1
Quantity/with_self/ustrip	1.55 ± 0.01 ns	1.55 ± 0.01 ns	1
QuantityArray/broadcasting/multi_array_of_quantities	0.15 ± 0.14 ms	0.149 ± 0.14 ms	1
QuantityArray/broadcasting/multi_normal_array	0.0498 ± 0.00022 ms	0.0498 ± 0.00023 ms	1
QuantityArray/broadcasting/multi_quantity_array	0.158 ± 0.00083 ms	0.158 ± 0.00086 ms	1
QuantityArray/broadcasting/x^2_array_of_quantities	27.6 ± 2.9 μs	26.4 ± 2.5 μs	1.05
QuantityArray/broadcasting/x^2_normal_array	5.8 ± 0.93 μs	4.87 ± 1.2 μs	1.19
QuantityArray/broadcasting/x^2_quantity_array	7.03 ± 0.28 μs	6.92 ± 0.28 μs	1.02
QuantityArray/broadcasting/x^4_array_of_quantities	0.0787 ± 0.00058 ms	0.0786 ± 0.00049 ms	1
QuantityArray/broadcasting/x^4_normal_array	0.0499 ± 0.00019 ms	0.0498 ± 0.00017 ms	1
QuantityArray/broadcasting/x^4_quantity_array	0.0591 ± 0.00017 ms	0.0592 ± 0.0002 ms	0.999
time_to_load	0.237 ± 0.00066 s	0.238 ± 0.0008 s	0.995

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).