Self-sufficient MinGW-based toolchain / packages

We currently rely on the `msys2` channel for providing a MinGW-based toolchain. The main usage of this is currently to build R and its packages. The `msys2` channel hasn't been updated for more than 3 years and we are also missing some native dependencies like `m2w64-pcre2` in it for the latest R release. To make conda-forge self-sufficient on Windows, we should get rid of the `msys2` channel dependency and build up our own set of packages for this.

There are two approaches I could think of, each with its different problems:

1. Build up our own MinGW toolchain from scratch and compile all packages from source. We could also make the compiler as a matrix so that we build some packages in the same recipe for MSVC and MinGW. The disadvantage here is that we will replicate all the work from MSYS2. On the other side this better fits into our infrastructure and would also allow us to apply our own modifications to the source build.
2. Repackage binaries from [MSYS2](https://www.msys2.org/) as the `msys2` channel has done in the past. This is the easier approach but we need to think of which packages should be included. All? Only a fixed list?

Approach 2 can probably be scripted mostly using the metadata from MSYS2 and would be the path to go for now to get things up and running.