Show bundled extensions under "Extensions" icon

[juliasilge]

We bundle a number of extensions via "builtInExtensions" but they don't show up by default under the "Extensions" icon:

Some of the extensions we bundle include Quarto and some Jupyter ones. You will see them there after you search for them one time, at which point they will be added to the counter of installed extensions. This is pretty confusing behavior and we should make a Positron-specific treatment for bundled extensions.

VS Code itself bundles only a very few extensions, like "JavaScript Debugger", and I can't seem to ever get them to show up under the "Extensions" icon, even with searching. They are not counted in the counter of installed extensions.

Let's also consider turning off "Recommended" for under the Extensions icon since we are not on the proprietary marketplace that provides those recommendations.

[juliasilge]

We are likely going to bundle more extensions, including the open source tunneling extension that VSCodium also bundles, so this is going to come up early for folks.