support register variable/command, add unit test

[AbaoFromCUG]

Register command mechanism

Regarding to Command, vscode has a mechanism named Command, which can be used as Command Variable

If the predefined variables from above are not sufficient, you can use any VS Code command as a variable through the ${command:commandID} syntax.
A command variable is replaced with the (string) result from the command evaluation. The implementation of a command can range from a simple calculation with no UI, to some sophisticated functionality based on the UI features available via VS Code's extension API. If the command returns anything other than a string, then the variable replacement will not complete. Command variables must return a string.

For example, vscode-cmake-tools extension support lots of command variable, which can be used 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}"
                }
            ]
        }
    ]
}

What did I do

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)

Some Unit Test Added

• normal variable
• env varialbe
• register command
• register asnyc command

[AbaoFromCUG]

Closed with additional inject code via dap.expand_config_variables