Improve Deno versions compatibility management

[umbopepato]

The README and documentation advertise a broad compatibility with Deno versions ("recent" and ^1.0.0). However, more than one Deno minor/patch release caused errors that required a dedicated update in vr. We should better specify the version compatibility ranges in the documentation and now that CLI installation helpers will introduce an even stricter dependence on deno CLI args we should probably print an warning when a new unsupported/untested version of Deno is used.