Modernize projects, fix memory leak, set working dir

[MV10]

As discussed in #32 this PR:

• Modernizes the solution / project formats
• Converts the library to .NET Standard 2.0 for legacy and modern .NET compatibility
• Updates DemoService to .NET Framework 4.8.1 (given 4.8 is the "forever" release of Framework)
• Adds DemoModernService showing the implementation under .NET 8.0 (current LTS release)
• Fixes the memory leak as described in #5
• Applies the working directory fix described on StackOverflow

I know I said earlier I'd do two PRs, but since the modernization didn't touch ProcessExtensions.cs at all, I figured it would be easy to review both in this single PR. In my real-world project where I used a copy of this code, I also found that I needed the StackOverflow working directory fix, so I went ahead with that as well.

[murrayju]

Looks great, thanks for putting this together!