Display local HTML files in Viewer

We're starting to gather more examples of folks wanting/expecting to see local HTML files in the Viewer:

- reprex, as reported in the private beta: https://github.com/posit-dev/positron-beta/issues/25
- Quarto output, as reported in Slack: https://positpbc.slack.com/archives/C05M2EZCPGR/p1711592538988499

For clarity, the recent work on the Viewer such as #2502 only handles HTTP URLs. 