* Create `CurrentEnv` and `MergeEnv` helper functions
* Add another test for `paths.RemoveFromPath` function
* Move executable finding functions to shims package
* Correct PATH code for env and exec commands
* 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