Fetch CMAKE variables from CMakePresets.json and CMakeLists.txt

### Brief Issue Summary

Are there any ways to fetch variables or options from CMAKE via command substitution, e.g. library path provided by package managers in toolchain file and presets? It's helpful in `launch.json` file.

### CMake Tools Diagnostics

```shell

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### Debug Log

```shell

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### Additional Information

_No response_

Start by reviewing how CMakePresets.json, CMakeLists.txt, and toolchain files expose variables or options for use in launch.json. Clarify the supported command-substitution behavior and define completion as reliably making the requested CMake values available in launch.json; the issue provides no diagnostics or tests.

Written by the indexing model from the issue text.