Add a command for reading cmake environment defined in CMakePresets.json

### Brief Issue Summary

vscode-cmake-tools provides command substitution which can be used in VSCode's tasks.json and launch.json, but all those substitutions are predefined variables. I have a custom environment variable defined in CMakePresets.json:
```json

"version": 2,
"configurePresets": [
{
"name": "default",
"environment": {
"MY_ENVIRONMENT_VARIABLE": "Test",
}
}
}
```

I want to refer `MY_ENVIRONMENT_VARIABLE` in tasks.json, but vscode-cmake-tools doesn't have a command for it.

### CMake Tools Diagnostics

_No response_

### Debug Log

_No response_

### Additional Information

_No response_

Start by locating the command-substitution support used by tasks.json and launch.json, then inspect how environments from CMakePresets.json are made available. The change is complete when a command can read MY_ENVIRONMENT_VARIABLE from the selected preset and use its value in those VS Code configurations.

Written by the indexing model from the issue text.