avoid resolving conflicts using "Open in External Editor" flow if no editors found

[shiftkey]

Overview

Closes #6060

Description

• lift error out of findEditorOrDefault, make App responsible for that, and return null if no editors can be found
• update MergeConflictsDialog to resolve the editor after mounting, and not show the button if the editor cannot be found
• test happy path when an editor can be found -> user can resolve conflicts in editor
• test sad path when not editors can be found -> buttons are disabled

Release notes

This is related to a new feature that hasn't yet made it to production

Notes: no-notes

[shiftkey]

Here's what the conflicts dialog looks like when we're unable to find any editor:

[shiftkey]

@billygriffin @ampinsk @outofambit just going to call out what I'd like input on from a UX side as part of addressing this bug:

• given all the whitespace on the right-hand column when you don't have an active editor, should we try and make use of that space? i'll see if i can put together a screenshot of what this looks like with long paths
• should we make the command line link more prominent - i.e. above the list of files - if that's what they need to use?

[billygriffin]

Recap of discussion with @shiftkey, @ampinsk, @tierninho, and me:

From @shiftkey's screenshot in #6109 (comment), we'll add in buttons as normal next to each conflict.

• The buttons will say "Open in editor," cased appropriately
• The buttons will be disabled
• Hovering over each button will have a tooltip that says: "No editor configured in Preferences > Advanced"

[shiftkey]

One extra thing:

"No editor configured in Preferences > Advanced"

This is a platform-specific label too (Preferences on macOS, Options on the others)

[shiftkey]

Some updated screenshots based on the feedback from above:

[tierninho]

LGTM!

[outofambit]

💯