Clean up StringAsTempFile files on SIGTERM, or preferably remove StringAsTempFile

[SteveSandersonMS]

There is logic that should, when the .NET process ends, also shut down the child Node process and remove any temporary files it created. However I haven't verified this works correctly across platforms.

[SteveSandersonMS]

Currently, StringAsTempFile will clean up its temp file in the following cases:

a. You Dispose it
b. Its .NET finalizer runs
c. (.NET Core) AssemblyLoadContext.Default.Unloading fires
d. (.NET Framework) ProcessExit or DomainUnload fires on AppDomain.CurrentDomain

These cover a lot of circumstances, but not all. For example, a SIGTERM, which is the graceful shutdown signal on macOS/Linux, does not cause AssemblyLoadContext.Default.Unloading to fire, so the temp files are left behind [*].

Ideally we should either:

• Find a more robust way of cleaning up the state, including on SIGTERM. I don't know of such a mechanism presently.
• Or, better, remove StringAsTempFile altogether. If there was some way for the NuGet package to extract its embedded .js file to disk when installed into a project, we could reference the file directly at that location. Unfortunately there doesn't appear to be a way of doing that right now, because of this bug with project.json tooling and waiting for the world to move to csproj.

[*] Obviously this applies to more severe shutdown scenarios like SIGKILL too, but it's excusable to leave state behind in that case.

[SteveSandersonMS]

This has been the case for a long time without causing anyone any problems, it already covers as many shutdown scenarios as we can, and we don't have any plausible proposal for how to cover any other scenarios (SIGKILL). So, closing this unless anyone actually observes a problem with it.