"terminal.integrated.env.linux" parsing improvements

Hello!

I regularly use "terminal.integrated.env.linux" to set additional paths, but using it inside a remote WSL is quite limited at the momment:

1. Setting PATH with "terminal.integrated.env.linux" overrides the /bin/remote-cli that was prepended breaking some integrations
   • https://stackoverflow.com/questions/74554169/how-do-i-modify-path-in-the-vscode-integrated-terminal-without-overwriting-vscod
   • Also mentioned in https://github.com/microsoft/vscode-remote-release/issues/9298
2. Vscode settings variables are not expanded as expected https://github.com/microsoft/vscode-remote-release/issues/35#issuecomment-490022164. So it's not possible to use ${workspaceRoot}/bin like in a native install, but ${env:PATH} works (it's missing .../bin/remote-cli as mentioned)
3. WSL config fails to load windows PATH Spaces in windows PATH break when appendWindowsPath is true
   • This seems to be on WSL, but worth mentioning as it's affecting PATH too
   • Btw I guess it would never load "terminal.integrated.env.windows" but it would be interesting as an option

# no setting: base path, notice bin/remote-cli
diegomateos@LTMITS180:~$ $PATH
bash: /home/diegomateos/.vscode-server/bin/ac4cbdf48759c7d8c3eb91ffe6bb04316e263c57/bin/remote-cli:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/usr/lib/wsl/lib:/snap/bin: No such file or directory

# setting: "terminal.integrated.env.linux": { "PATH": "${env:PATH}:${workspaceRoot}/bin", },
# WARN: lost bin/remote-cli, ${env:PATH} expanded but ${workspaceRoot} nope
diegomateos@LTMITS180:~$ $PATH
bash: /usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/usr/lib/wsl/lib:${workspaceRoot}/bin:/snap/bin: No such file or directory

TBH I actually use local nested settings inside the config which is not very documented but works quite well.
The expansion seems related to vscode vars like ${workspaceRoot}, so hopefully these would work too.

// minimal example of local nested vars (names are arbitrary): you can reuse / merge them and can be used in launch/tasks too
"env": {
     "tools":"${workspaceRoot}/_tools",
     "lin": {
          "path" : "${config:env.tools}:${env:PATH},
      }
}
"terminal.integrated.env.linux": { "PATH": "${config:env.lin.path}", },

Thanks, best regards!

No repository files, tests, or implementation entry points are named. Start by reproducing the terminal.integrated.env.linux cases in a remote WSL workspace, then trace how PATH and VS Code variables are expanded; done means the requested PATH preservation and variable parsing behavior is defined and covered for the reported scenarios.