Update manifest.json R version population

[m--]

We should update how we decide which R version we specify for running on Connect. This work would update the the R.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 R.version in the config, if set (this field is now optional)
2. From the current active version of R in the IDE, if the R.version is not set in config

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

Note that this doesn't lean on the R version defined in renv.lock, which, ultimately, we may want to do. If we tackle that change, it will be in a follow-on issue.