make the extension use VS Code LM API

[amithegde]

With LM API it can connect to the models provided by github copilot. Currently the extension asks for LLM API key

[ultmaster]

Check out the prompt gallery feature: https://microsoft.github.io/poml/latest/vscode/features/#prompt-gallery You can register a poml file and use @poml to invoke the prompt.

I've been thinking about whether it's possible to add the feature in poml.test command, but I think it will cause more inconsistencies because vscode's built-in LLM really only supports a limited subset of commonly acknowledged LLM features. I put that in a low priority list.