Add `exec-env` template with `TOOL_ROOT` env

**Is your feature request related to a problem? Please describe.**

Many plugins don't set `TOOL_ROOT` env variable(s), see hensou/asdf-dotnet#26, asdf-community/asdf-rust#32, rkyleg/asdf-julia#36; some used to set it [the wrong way](https://github.com/halcyon/asdf-java/commit/b0de93a961a5392de4bd1e790e62b5e451a9041b)

**Describe the solution you'd like**

I'd be nice to add an `exec-env` template

**Describe alternatives you've considered**

Adding information about it into the `README`, but it seems less convenient and can be easily missed

**Additional context**

Inspect the repository's existing plugin-template layout and the README, which the issue identifies as an alternative documentation location. Determine where an exec-env template belongs and what generated content should expose TOOL_ROOT; done means the template is included and its purpose is discoverable without relying only on README text.

Written by the indexing model from the issue text.