feat!: replace mustache with nunjucks variable rendering

[FidelusAleksander]

Summary

Important

This will introduce a new major, v3 release of action-text-variables

This pull request introduces significant updates to the repository, transitioning from Mustache to Nunjucks for template rendering, enhancing functionality with advanced templating features, and updating associated workflows, tests, and documentation to reflect these changes. The most important changes include integrating Nunjucks, updating templates and tests, and modifying workflows for improved variable handling.

Breaking changes and migrating

Mustache and nunjucks both support the {{ variable }} syntax. So for all usages of this action that only used the {{ variable }} syntax, this change should be a drop-in replacement.

Mustache and nunjucks differ in the way they support conditional logic, loops and other advanced templating features. Refer to the Nunjucks documentation for more information on how to use these features

In general, Nunjucks offers a broader range of features and is more powerful than Mustache.

Closes: #39

Task list

• For workflow changes, I have verified the Actions workflows function as expected.
• For content changes, I have reviewed the style guide.

[Copilot]

Pull Request Overview

This PR updates the GitHub Action to use Nunjucks for variable rendering instead of Mustache.

• Replaces Mustache with Nunjucks in the core logic and updates corresponding dependencies in package.json, action.yml, and documentation.
• Updates tests and CI workflows to verify new Nunjucks features (e.g. conditional content, loops, filters).

Reviewed Changes

Copilot reviewed 12 out of 16 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/main.js	Core update to use nunjucks.renderString instead of mustache.render.
package.json	Dependency change from mustache to nunjucks.
action.yml	Metadata description updated to reflect nunjucks usage.
tests/test-template.md	New test template showcasing various nunjucks features.
tests/sample-template.md	Removed outdated sample template.
tests/main.test.js	Test suite updated to verify nunjucks variable rendering and features.
README.md	Documentation updated to reference nunjucks.
.prettierignore	Updated to ignore test-template file.
.github/workflows/ci.yml	CI workflows updated to use the new test template and Nunjucks features.
.github/linters/eslint.config.mjs	ESLint ignore patterns updated.
.github/linters/.markdown-lint.yml	Markdown lint rules updated.
.github/copilot-instructions.md	Instructions updated to reflect nunjucks variable replacement.

Comments suppressed due to low confidence (1)

src/main.js:47

• The test template uses a 'title' filter, but nunjucks does not register a custom 'title' filter by default. Consider registering a custom 'title' filter in the configuration (e.g., using nunjucks.configure({...}).addFilter('title', value => /* transformation logic */)) to ensure that the filter behaves as expected.

    const output = nunjucks.renderString(templateText, templateVars)

[arilivigni]

I think the Copilot comment is a nitpick.
LGTM. Great job!

[chriswblake]

I'm pretty good with this. Only a few minor comments about the unit tests.

[FidelusAleksander]

@chriswblake Split the unit tests into multiple files, kept one template for simplicity for now - we can break it down in separate PR if we want

Updated test case naming to be more cohesive