* Create `CallbackPath` method on `Plugin` struct
* Correct behavior of `asdf shimversions` command
* Update `shims.FindExecutable` function to return plugin
* Create `repotest.WritePluginCallback` function
* Create `execenv` package for invoking `exec-env` plugin callback
* Make `MapToSlice` a public function
* Return resolved version from `shims.FindExecutable` function
* Create `shims.ExecutablePaths` function
* Enable `shim_env_command.bats` tests
* Implement `asdf env` command