Update manifest.json Quarto version population

[sagerb]

We should update how we decide which Quarto version we specify for running on Connect. This work would update the Quarto.version field in the config to be optional, and document what it means to set it. This should also update how we build the manifest.json accordingly:

1. From the Quarto.version in the config, if set (this field is now optional)
2. From the current active version of Quarto in the IDE, if the Quarto.version is not set in config

The answer to this should populate the quarto.version field in the manifest.json, when we build it.