Close open settings files before download overwrite

[apples-kksk]

Fixes #396.

Summary

• Close visible editors for files that are about to be overwritten during download sync.
• Normalize file paths before matching open editors, including case-insensitive matching on Windows.
• Add a VS Code extension-host test for closing an open settings file.

Verification

• npm run tslint-check
• ./node_modules/.bin/tsc -p ./
• npm run compile
• npm run vscode:prepublish
• CODE_TESTS_DATA_DIR="$(mktemp -d)" xvfb-run -a node ./node_modules/vscode/bin/test

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IssueHunt Summary

Referenced issues

This pull request has been submitted to:

• #396: Upload/Download Conflicts

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