Add playwright tests to vscode-superoffice-core

[ejfasting]

Playwright is not able to run unit tests directly, as 'vscode' is not a node_modules package.
Playwright should be able to run vscode, as its an electron-app, and from there do all the tests in the extension.

Some resources for inspiration:
https://github.com/microsoft/playwright-vscode/pull/353/files#diff-42301b1071aa17169b368e51222b775dd5d5e19649ff424f5fb1d12b4132b84c

https://github.com/microsoft/vscode/blob/62cb62c07bd4646fa1fe9929e82fba32e81976a7/test/automation/src/playwrightElectron.ts#L29