[task] add Playwright end-to-end tests to the pipeline

Problem/Motivation

Source settings forms are rebuilt over AJAX in every plugin type that embeds a component form: block, field formatter, layout, views style, ui_patterns_ui (and later field widgets, ckeditor...etc).
Nothing covers this today and we want to find the remaining issues, correct them, as well as prevent regressions.

Proposed resolution

Core is moving from Nightwatch to Playwright, gitlab_templates has no Playwright job yet, projects wire their own.
We will reuse the Playwright setup of display_builder, same fixtures and CI jobs, as a blocking job with green tests only.

We start with one spec on the block form: switch a prop source, nest a component in a slot, embed a block plugin form, remove a source, save, reopen, check everything persisted.

Remaining tasks

- MR with the setup, the first spec and the CI jobs.
- Then one spec per plugin type (in this issue or in follow-up issue): field formatter, layout, views, ui_patterns_ui...