[playwright] does not export TheiaTerminal and TheiaWelcomeView

Hi everyone, 

I noticed that the `TheiaTerminal` and `TheiaWelcomeView` are not exported from the Theia Playwright utilities.
To solve this, the `@theia/playwright/src/index.ts` file should export:

```ts
export * from './theia-terminal';
export * from './theia-welcome-view';
```

I also noticed that __theia-monaco-editor.ts__ is not exported. May I create a PR and add those exports?

