feat: add process.getSystemVersion() (backport: 5-0-x)

[trop]

Backport of #16599

See that PR for details.

Notes: Added process.getSystemVersion(), which is available in sandboxed renderers and returns the actual operating system version instead of kernel version on macOS.

[nornagon]

this needs approval from @electron/wg-releases

[jkleinsc]

The releases working group decided that this PR should not be merged back to 5-0-x.

[miniak]

@jkleinsc can you share more details about the decision?

[jkleinsc]

@miniak the decision was made because of a couple of things:

1. We decided that the default answer as to whether or not to add a feature once beta starts should be "no". This aligns with our versioning documentation: https://github.com/electron/electron/blob/master/docs/tutorial/electron-versioning.md#beta-releases-and-bug-fixes
2. There didn't seem to be a clear need for this feature to be added to 5-0-x. If you have a strong case as to why this should be added we might reconsider. In the future we are going to request that anyone who has a feature they want to backport to a beta will need to provide their argument as to why the feature should be backported.