Use 3-way merge editor to view and edit SVN merge conflicts

[FractalBoy]

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This pull request implements a command for opening merge conflicts in VSCode's 3-way merge editor.

Currently, this editor is only used by the git extension and the command is technically private (_open.mergeEditor). However, it works and does the same thing that code -m does on the command line.

If you have a merge conflict and you click it in the list of changes, it will now open this merge editor instead of just opening the file.

[FractalBoy]

It looks like there was a bunch of outdated stuff causing test failures. I went ahead and resolved all of those issues so that the required checks can pass.

[FractalBoy]

@JohnstonCode any chance you can review this PR?

[ParticleG]

This project seems to no longer be maintained