Fix reflection by using `dimension_names`

[MilesCranmer]

Seems sometimes inference issues propagate to the all_dimensions function's use of Base.fieldnames. Switching to the internally-defined dimension_names fixes it.

TODO: verify this does not affect performance!

[github-actions[bot]]

Benchmark Results

	main	b1ae402e89a831...	main/b1ae402e89a831...
Quantity/creation/Quantity(x)	2.79 ± 0.009 ns	2.79 ± 0.009 ns	1
Quantity/creation/Quantity(x, length=y)	3.41 ± 0.01 ns	3.41 ± 0.01 ns	1
Quantity/with_numbers/*real	3.1 ± 0.01 ns	3.1 ± 0.01 ns	1
Quantity/with_numbers/^int	8.05 ± 2.2 ns	8.05 ± 2.2 ns	1
Quantity/with_numbers/^int * real	8.37 ± 1.9 ns	8.37 ± 1.9 ns	1
Quantity/with_quantity/+y	4.04 ± 0.01 ns	4.04 ± 0.001 ns	1
Quantity/with_quantity//y	3.11 ± 0 ns	3.11 ± 0 ns	1
Quantity/with_self/dimension	3.11 ± 0.01 ns	3.11 ± 0.001 ns	1
Quantity/with_self/inv	3.11 ± 0.001 ns	3.11 ± 0.001 ns	1
Quantity/with_self/ustrip	2.79 ± 0 ns	2.79 ± 0.009 ns	1
QuantityArray/broadcasting/multi_array_of_quantities	0.143 ± 0.00064 ms	0.146 ± 0.0009 ms	0.979
QuantityArray/broadcasting/multi_normal_array	0.0559 ± 0.0029 ms	0.0559 ± 0.0029 ms	1
QuantityArray/broadcasting/multi_quantity_array	0.155 ± 0.00082 ms	0.155 ± 0.00064 ms	1
QuantityArray/broadcasting/x^2_array_of_quantities	24.5 ± 2.5 μs	24.1 ± 2.3 μs	1.02
QuantityArray/broadcasting/x^2_normal_array	4.79 ± 0.95 μs	5.43 ± 0.66 μs	0.882
QuantityArray/broadcasting/x^2_quantity_array	7.1 ± 0.29 μs	7.02 ± 0.26 μs	1.01
QuantityArray/broadcasting/x^4_array_of_quantities	0.0816 ± 0.00064 ms	0.0815 ± 0.00052 ms	1
QuantityArray/broadcasting/x^4_normal_array	0.0499 ± 0.00017 ms	0.0499 ± 0.00018 ms	1
QuantityArray/broadcasting/x^4_quantity_array	0.053 ± 0.00022 ms	0.053 ± 0.00021 ms	1
time_to_load	0.145 ± 0.0023 s	0.145 ± 0.00088 s	0.998

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).