Allow mac and linux users to preconfigure a directory with extensions to be preinstalled

[melissa-barca]

This was implemented for Windows only in 1.96: https://code.visualstudio.com/updates/v1_96#_set-up-vs-code-with-preinstalled-extensions

In #6302 we are expanding this functionality to install extensions that are prepackaged with Positron using this feature. We should consider if we want to expand our feature to allow Mac and Linux users to configure their own directory with VSIX files to install in Positron.