playwright: add page object for terminal

[planger]

What it does

With this PR, we add a Playwright page object to interact with the terminal. The terminal (xterm) uses a canvas to show the terminal contents. Thus, we use a workaround of selecting all contents and copying into the clipboard to read the output of the terminal.

Contributed on behalf of STMicroelectronics

How to test

Aside from making sure the added test runs in the CI, it'd be great to test this also on Mac and Windows locally. I confirmed that it works on Linux.

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[marcdumais-work]

@planger Sorry for the delay in reviewing this PR. Looks good so far. I have taken the liberty of enabling the Playwright test suite on all OS by adding a temporary commit on top of your contribution and pushing the result on my fork:

https://github.com/marcdumais-work/theia/actions/runs/4679291140

Windows and macOS have failed: Windows has an issue with the way THEIA_CONFIG_DIR is set in playwright package.json. On Mac, there's a failure in theia-text-editor.test.ts, but the new tests have passed. So, I think this PR does not make things worse - if we want to run these tests on the other OS, there will be a bit of work, not directly related to this contribution.

[marcdumais-work]

LGTM - thanks @planger! The added infrastructure will make it easy to add more complex terminal tests later if needed.

[marcdumais-work]

Just FYI, it's not wrong to update the Copyright date, but as per EF guidance, we consider that git is the source of truth, and so generally do not require this be done when an existing file is modified.

[planger]

Thanks a lot for the review @marcdumais-work! I've created #12405 to track the issue on Windows and Mac.