[lldb-dap] Support variable substitution in VS Code extension

The LLDB DAP VS Code extension should support variable substitution, so the settings can be configured like this:
```JSON
{
"lldb-dap.executable-path": "${env:ProgramFiles}\\LLVM\\bin\\lldb-dap.exe",
"lldb-dap.environment": {
"Path": "${env:LOCALAPPDATA}\\Python\\pythoncore-3.10-64;${env:Path}"
}
}
```
It will be very useful to have this feature. Many other VS Code extensions, like `Clangd` and `CMake Tools`, support it.

Start by locating the LLDB DAP VS Code extension code that reads lldb-dap.executable-path and lldb-dap.environment, then inspect how those settings are passed to lldb-dap. Use the issue's ${env:...} examples to define the expected resolved executable path and environment values, and verify the behavior with the extension's available tests or a manual VS Code configuration.

Written by the indexing model from the issue text.