✨ Add support for compiling Jinja-style templates with Nunjucks in sy…

…nc process (#271)

* Add `nunjucks` and update `@actions/core`

* Read template info from YAML config file

* Render templates when copying files

* Compile JS files into action

* Add documentation about the templating feature

* update readme

Co-authored-by: Maximilian Schiller <schiller@mxis.ch>

README.md

@@ -20,6 +20,7 @@ With [repo-file-sync-action](https://github.com/BetaHuhn/repo-file-sync-action)
 - Create a pull request in the target repo so you have the last say on what gets merged
 - Automatically label pull requests to integrate with other actions like [automerge-action](https://github.com/pascalgn/automerge-action)
 - Assign users to the pull request
+- Render [Jinja](https://jinja.palletsprojects.com/)-style templates as use variables thanks to [Nunjucks](https://mozilla.github.io/nunjucks/)
 
 ## 📚 Usage
 
@@ -202,6 +203,54 @@ user/repo:
     replace: false
 ```
 
+### Using templates
+
+You can render templates before syncing by using the [Jinja](https://jinja.palletsprojects.com/)-style template syntax. It will be compiled using [Nunjucks](https://mozilla.github.io/nunjucks/) and the output written to the specific file(s) or folder(s).
+
+Nunjucks supports variables and blocks among other things. To enable, set the `template` field to a context dictionary, or in case of no variables, `true`:
+
+```yml
+user/repo:
+  - source: src/README.md
+    template:
+      user:
+        name: 'Maxi'
+        handle: '@BetaHuhn'
+```
+
+In the source file you can then use these variables like this:
+
+```yml
+# README.md
+
+Created by {{ user.name }} ({{ user.handle }})
+```
+
+Result:
+
+```yml
+# README.md
+
+Created by Maxi (@BetaHuhn)
+```
+
+You can also use `extends` with a relative path to inherit other templates. Take a look at Nunjucks [template syntax](https://mozilla.github.io/nunjucks/templating.html) for more info.
+
+```yml
+user/repo:
+  - source: .github/workflows/child.yml
+    template: true
+```
+
+```yml
+# child.yml
+{% extends './parent.yml' %}
+
+{% block some_block %}
+This is some content
+{% endblock %}
+```
+
 ### Delete orphaned files
 
 With the `deleteOrphaned` option you can choose to delete files in the target repository if they are deleted in the source repository. The option defaults to `false` and only works when [syncing entire directories](#sync-entire-directories):