Use `wdio-vscode-service` for e2e tests

[mattseddon]

Problems to solve

• Add wdio-vscode-service with simple tests
• Build webview page object selector
• Test that webview is open and working
• Structure job (needs to build venv)
• Shared VS Code instance with .vscode-test - won't do (cost/benefit)
• Investigate screenshot regression tests - need somewhere to store/compare baseline
• If using screenshots - work out how to update the baseline
• Schedule for running in CI

Decided against messing around with screenshots for the time being.

[mattseddon]

Relates to #429

[mattseddon]

Example usage: stateful/vscode-marquee#67