Fix up some logging (Quarto CLI discovery, extension host warnings)

[juliasilge]

This PR addresses:

• the first part of Log Quarto CLI discovery in Quarto output channel #846 (logging where/how the Quarto CLI is found)
• Quarto extension pollutes Extension Host output channel with a lot of warnings #443 which is soooooo annoying when dealing with the output channels

After these changes, you'll see logs like this in the Quarto output channel, depending of course on the exact details of how your Quarto is configured:

I went fairly verbose on these logs and am open to feedback about this being overkill, but I tend to think more is better, at least to some level. I did not add any of this to the status bar or anything like that; I think that the output channel is a good spot for folks to look for this kind of info.