Results view: Don't reopen webview if it's already visible

[shati-patel]

A user reported a bug where the results view is opened in the active editor, even if the results view is already visible in a different column:

🖱️ Expand for example GIF

This PR fixes that behaviour. I.e. when there's already a results view visible (not necessarily active), we display the results in that panel.

Notes

A slight weirdness we could still find is that if the results view is open in a tab somewhere, but not visible, it can still move to a different place. I don't think this is a problem, but I can open an issue if it is!

🖱 Expand for details

In this case, the results view is open on the right, but it isn't currently visible. Since the right-hand side is the active editor, the new results open on the left. (So the results view moves column.)

Checklist

• CHANGELOG.md has been updated to incorporate all user visible changes made by this pull request.
• [n/a] Issues have been created for any UI or other user-facing changes made by this pull request.
• [n/a] If this pull request makes user-facing changes that require documentation changes, the ready-for-doc-review label has been added to this pull request or the corresponding issue.