Add template for upload to asset library

I overlooked the section of the documentation for
https://github.com/deep-entertainment/godot-asset-lib-action which says
that you have to provide a template.

Personally I don't see why a built-in template couldn't be provided: all
fields in the request are optional and default to their existing values;
and if you are using this action you presumably want the normal
GitHub-based export. But this way we can keep the description etc. in
version control.

https://phabricator.endlessm.com/T35503

Author: wjt

.asset-template.json.hb

@@ -0,0 +1,14 @@
+{
+  "title": "Block Coding",
+  "description": "Create games using a high-level, block-based visual programming language.\r\n\r\nIntended as an educational tool for learners in the earlier stages of their journey towards becoming game developers. This plugin lets you create your first games with high-level blocks, avoiding the immediate need to learn to code in GDScript. Building games in this way provides a gentle introduction to programming concepts and allows you to focus your efforts on becoming familiar with the rest of the Godot Editor UI.",
+  "category_id": "5",
+  "godot_version": "4.2",
+  "version_string": "{{ context.release.name }}",
+  "cost": "MIT",
+  "support_level": "community",
+  "download_provider": "GitHub",
+  "download_commit": "{{ context.release.tag_name }}",
+  "browse_url": "{{ context.repository.html_url }}",
+  "issues_url": "{{ context.repository.html_url }}/issues",
+  "icon_url": "https://raw.githubusercontent.com/endlessm/godot-block-coding/{{ context.release.tag_name }}/icon.png"
+}