Plugin template doesn't follow asdf "Golden Rules for Plugin Scripts"

**Describe the bug**

The plugin template is violating the asdf project's "[golden rules](https://asdf-vm.com/plugins/create.html#golden-rules-for-plugin-scripts)" for portability

Most specifically it's using `echo` and `source` liberally, and these have been considered "banned commands" for more than a year. (I understand these are bash scripts and can make assumptions but... still!)

**Steps to reproduce**

read the source, or generate the template

**Expected behavior**

No golden rule violations. In fact I'd expect the template to be enforcing this as part of linting?

**Screenshots**

**Additional context**

https://github.com/asdf-vm/asdf/blame/fce6c534daf6c0b2eed7dc633b18f7b87f665d2f/test/banned_commands.bats#L5-L24

Read the plugin template source and the referenced asdf test/banned_commands.bats rules first, then inspect how the template is generated and linted. Update the template so its scripts follow the listed portability rules and verify that generated scripts contain no banned commands.

Written by the indexing model from the issue text.