fix(powershell): run `atuin history end` in the background (#3034)

This runs `atuin history end` in the background, since it could delay
the next prompt when syncing.

Sorry I didn't realize this command could be slow (I currently don't use
sync).

I went for the .NET [`Process`
class](https://learn.microsoft.com/en-us/dotnet/api/system.diagnostics.process),
which is not pretty, but the other solutions I thought about all had
their own issues (`Start-Process` needs a file to redirect the output so
you can't just ignore it, `Start-Job` creates job objects that will
linger, `&` does the same but doesn't work on PS 5.1, ...). I'm
surprised I couldn't find a nice way to do the equivalent of `command &
>/dev/null` in PowerShell. 😕

This replaces #3033

## Checks
- [x] I am happy for maintainers to push small adjustments to this PR,
to speed up the review cycle
- [x] I have checked that there are no existing pull requests for the
same thing

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