Support Command variables in dap

[AbaoFromCUG]

I am sorry that I don't read the Contribute section in README.md

Before making direct code contributions, please create a discussion or issue to clarify whether the change is in scope of the nvim-dap core.

I had push a pull request, #996, which is about Register command mechanism

I add API named dap.register_command(identifier, handler) to do something same to vscode.commands.registerCommand . Which can help extension developer to enhance ability of dap.

e.g.

require("dap").register_command("cmake.launchTargetPath", function()
   // run cmake build task, then return executable path
   // can be a asynchronous coroutine
   return coroutine.create(function()
         // fake cmake build step
         vim.defer_fn(function()
            coroutine.resume(co, "/path/to/exectutable")
          end, 2000)
  end)
end)

With this change, we can use comand variable in launch.json

{
    "version": "0.2.0",
    "configurations": [
        {
            "name": "(gdb) Launch",
            "type": "cppdbg",
            "request": "launch",
            // Resolved by CMake Tools:
            "program": "${command:cmake.launchTargetPath}",
            "cwd": "${workspaceFolder}",
            "environment": [
                {
                    // add the directory where our target was built to the PATHs
                    // it gets resolved by CMake Tools:
                    "name": "PATH",
                    "value": "${env:PATH}:${command:cmake.getLaunchTargetDirectory}"
                }
            ]
        }
    ]
}

Whether the change is in scope of the nvim-dap core?

Say sorry again