feat: Support variable substitution in VSCode settings
Currently support a subset of [variables provided by VSCode](https://code.visualstudio.com/docs/editor/variables-reference) in `server.extraEnv` section of Rust-Analyzer settings:
* `workspaceFolder`
* `workspaceFolderBasename`
* `cwd`
* `execPath`
* `pathSeparator`
Also, this PR adds support for general environment variables resolution. You can declare environment variables and reference them from other variables like this:
```JSON
"rust-analyzer.server.extraEnv": {
"RUSTFLAGS": "-L${env:OPEN_XR_SDK_PATH}",
"OPEN_XR_SDK_PATH": "${workspaceFolder}\\..\\OpenXR-SDK\\build\\src\\loader\\Release"
},
```
The order of variable declaration doesn't matter, you can reference variables before defining them. If the variable is not present in `extraEnv` section, VSCode will search for them in your environment. Missing variables will be replaced with empty string. Circular references won't be resolved and will be passed to rust-analyzer server process as is.
Closes#9626, but doesn't address use cases where people want to use values provided by `rustc` or `cargo`, such as `${targetTriple}` proposal #11649
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