Add command for opening an HTML file in a browser

[jmcphers]

This change adds an "Open in Browser" command to HTML files to go with "Open in Viewer", based on user feedback that it is not obvious how to open an HTML file in a browser if you're looking at it in Positron.

Addresses #6047.

Release Notes

New Features

• New "Open in Browser" command for HTML files (Provide an affordance for opening HTML files directly in a web browser #6047)

Bug Fixes

• N/A

QA Notes

• This change exists only in the Positron Proxy extension and thus should not affect any core scenarios.
• I changed the icon for the "Open in Viewer" command to "preview" since I think it's better suited to the gesture.

[github-actions]

E2E Tests 🚀
This PR will run tests tagged with: @:critical

^{readme  valid tags}

[petetronic]

I tried out the new command and the new Open in Browser button appeared as expected and worked to open .html files in my default browser. However, using Cmd+Shift+P to select the command "Open in Browser", I got an error:

[jmcphers]

Thanks, I had not anticipated the use of this command from the Palette! That should work now, as long as you've got an HTML file open in the editor (otherwise you should get a friendly error).

[softwarenerd]

LG!