Provide a more extensive process API

[alexrp]

Since we're banning System.Diagnostics.Process and providing an API-incompatible replacement, it means people will not be able to use libraries like MedallionShell.

We should provide a similarly rich API to make up for that fact.

Here are some of the main things I think I'd like to change:

• [x] When a process output stream is redirected, we should buffer up all the output so it can be read at any point. This is going to be a reasonable default for the vast majority of cases.
• [x] ~There should be a way to redirect process output to the moral equivalent of /dev/null.~
• [x] ~Provide convenient piping syntax with overloaded |, >, and < operators.~
• [x] TerminalProcess.SendSignal should throw an exception if the given signal cannot be synthesized (e.g. CTRL_CLOSE_EVENT on Windows).
• [x] Drop support for process diagnostic inspection properties. Most of this stuff is poorly implemented outside of Windows and rarely used.
• [x] Translate non-zero exit codes to an exception (with an option to turn this off).
• [x] Rename TerminalProcess to ChildProcess and move it to System.Processes namespace.

[alexrp]

Also address this comment:

https://github.com/alexrp/system-terminal/blob/9a688a9b3f088a06ab1f8be2de07eb19c3a2932c/src/core/Diagnostics/TerminalProcess.cs#L5

[alexrp]

• There should be a way to redirect process output to the moral equivalent of /dev/null.
• Provide convenient piping syntax with overloaded |, >, and < operators.

I'll spin these off into a separate issue.