Support variable substitution in vscode settings

[Veykril]

VSCode settings variables like ${workspaceFolder} don't get substituted by VSCode in extension settings, so when people try to use these in RA settings they will run into unfortunate surprises, see #4616 (comment).

There is an open issue on the VSCode tracker here, microsoft/vscode#2809 (comment), but it doesn't look like this will be solved in the near future.

So it would be good for us to do the substitution ourselves where applicable as to not surprise vscode users. The list of vars we want to support is here https://code.visualstudio.com/docs/editor/variables-reference#_predefined-variables, not necessarily all of them, but those that make the most sense at least, if possible.

[Veykril]

I just realized #12215 only implemented this for the server env, not for the entirey of configs.

[Veykril]

env part tracked in #14053