feature: Consider adding `asdf` for Neovim version matrix testing in CI

[ColinKennedy]

Did you check the docs?

• [X] I have read all the docs

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

I like this template and use it for my plugins. It'd be nice if the default CI tests against several neovim versions at the same time, especially since so much API things change in Neovim's nightly version and between minor versions (though we haven't gotten a new Neovim minor in a while). The template does stable and nightly but it'd be good to be able to attribute "this plugin works for X.Y specific Neovim versions" and have the CI directly reflect that.

Describe the solution you'd like

Add Neovim version matrix testing. It doesn't have to be asdf but I found a guide that could be useful - https://www.petergundel.de/asdf/ci/github%20actions/matrix%20testing/neovim/2023/11/11/leveraging-asdf-in-github-actions-ci-for-matrix-testing.html

Describe alternatives you've considered

n/a. Maybe https://github.com/MordechaiHadad/bob is possible for neovim installs. Not sure

Additional context

No response

[ColinKennedy]

Read through rhysd/action-setup-vim again. Actually that one is generally better. But I think this template should include multiple OSes like this workflow https://github.com/ColinKennedy/spellbound.nvim/blob/main/.github/workflows/test.yml

---
on: [pull_request]
name: test

jobs:
  test:
    strategy:
      matrix:
        os: [ubuntu-latest, macos-latest, windows-latest]
        neovim: [v0.10.0, nightly]
    runs-on: ${{ matrix.os }}
    name: "OS: ${{ matrix.os }} - Neovim: ${{ matrix.neovim }}"
    steps:
    - name: Checkout spellbound.nvim
      uses: actions/checkout@v4
      with:
        repository: ColinKennedy/spellbound.nvim
        path: spellbound.nvim
    - uses: rhysd/action-setup-vim@v1
      with:
        neovim: true
        version: ${{ matrix.neovim }}
    - name: Run tests
      working-directory: ./spellbound.nvim
      run: |
        nvim --version
        make test