Extend core templates

The selection of templates for `core` branch is quite limited.

I would expect at least
- F# templates on par with C#
- Precompiled functions templates

I can see that templates are loaded from `https://functionscdn.azureedge.net/public/templates.json`.
- Is there a way to send a pull request to this file?
- Is there a way to substitute this file with my own (for personal use / demos)?

Start by tracing how core templates are loaded from https://functionscdn.azureedge.net/public/templates.json and inspect the template definitions relevant to the core branch. Done means providing F# templates comparable to C#, adding precompiled-function templates, and clarifying whether contributors can update or substitute the template source.

Written by the indexing model from the issue text.