Positron Assistant: Web compatibility

[jmcphers]

This change adds web compatibility and other improvements to Positron Assistant:

• Disable the default vscode-api-tests default chat provider so that Positron Assistant can become the default chat provider in web dev mode (EDIT: It looks like this breaks the tests, so more work is needed here)
• Store API keys in global storage, not secret storage, in web mode. This necessary because secret storage is not durable in web mode, so API keys are lost on reload. Using global storage isn't optimal, and should be revisited. See https://github.com/rstudio/vscode-server/issues/174.
• Add Positron Assistant icon from Paul.
• Use global storage, not settings JSON, to store the list of registered models. I don't think settings makes sense here because they are not really user editable.
• Start adding quickpick to show registered models; this will eventually also let you delete one. We may upgrade this to a dedicated dialog later.

Some of this is still wet cement, mostly opening this to keep development lines open.

[github-actions]

E2E Tests 🚀
This PR will run tests tagged with: @:critical

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