Missing autocomplete support for prompt fragments in prompt templates

[sgraband]

Bug Description:

In Theia, the prompt templates lack autocomplete support in the prompt template editor. Only the 'prompt:' part of the text is suggested.

Steps to Reproduce:

1. Open the Theia editor.
2. Start to write "{{pr" in a prompt template.
3. Notice that while the 'prompt:' part of the text is auto-completed, no suggestions are given for the available prompt templates.

Additional Information

• Operating System: Ubuntu 24.04
• Theia Version: 1.59.0

[eneufeld]

I think we should combine this ticket with #15215