Update to use source based coverage instead of line based

[carolynzech]

Updates the extension to use source-based coverage instead of line-based coverage. See #139 for screenshots of how it looks.

1. a2e37a5 just applies the changes from Add source based coverage for the extension #139 in a single commit, rebased on top of main.
2. e2e3a40 changes the extension to show the "Get coverage info" button automatically. Before, our documentation instructed a user to go to settings to enable the button. I couldn't find the setting to do that, and I figured if I couldn't find it, our users wouldn't be able to either. I also don't think it makes sense to hide it behind a setting. We can print the warning that it's unstable, but the green/red coverage visual is one of the strongest arguments for using the Kani extension instead of the command line, so we should have this feature front and center.
3. 69f376b just removes comments and unused code.
4. 8a99568 updates the documentation to reflect the shift from line-based to source-based coverage, and the removal of the toggle option from e2e3a40.
5. 05d0246 introduces more custom types, so that we use those instead of any types.