Plugin E2E: Remove custom selector engine

[sunker]

What this PR does / why we need it:

This PR removes the custom selector engine and also updates some tsdocs.

Instead of using the custom selector engine, it's recommended to use the getByGrafanaSelector method that is exposed in all Grafana pages.

// instead of this:
page.locator(`selector=data-testid code editor`);
// do this:
panelEditPage.getByGrafanaSelector('data-testid code editor')

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

📦 Published PR as canary version: Canary Versions

✨ Test out this PR locally via:

npm install @grafana/[email protected]
# or 
yarn add @grafana/[email protected]

[github-actions]

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[jackw]

LGTM! 🚀