Support for running scripts in separate process

[jthomasmock]

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We have integrated testing and build processes into alternate panes or background processes as mentioned in #1303

However, we don't fully support running arbitrary user code in separate processes but the same runtime, i.e., Background Jobs from RStudio. Given that we only have one console, it feels a bit awkward to need to drop into the terminal and lose all of the default behavior to execute long-running scripts in the background.

These may not be as necessary given Python apps/APIs are typically run via the terminal and the Shiny extension runs R/Python via the terminal but it does feel like a gap compared to RStudio.

I also saw this recently of a VS Code R user asking for the jobs R package to work with VS Code: https://twitter.com/strengejacke/status/1791224380602622355