floating point matchers for std::complex<T>

**Description**
Describe the feature/change you request and why do you want it.

Our [application](https://github.com/QMCPACK/qmcpack) needs to compare complex numbers for equality. We created a [ComplexApprox class](https://github.com/QMCPACK/qmcpack/blob/2db0b25e4ad64b06f11cfb5ba9644c553094571a/external_codes/catch/complex_approx.hpp) to provide this functionality.

```
std::complex i(0.,1.), j(0.,1.);
CHECK(i == ComplexApprox(j));
```

We want to make an upstream contribution. Since [Approx is deprecated](https://github.com/catchorg/Catch2/issues/1444) could a custom matcher provide the same functionality? Is such a pull request likely to be accepted?