feat(create-plugin): make Playwright config extendable

[alectimison-maker]

What this PR does / why we need it:

• Moves the Playwright defaults generated by create-plugin into the managed .config/playwright.config.ts.
• Keeps the root playwright.config.ts as a small, user-owned override using defineConfig(baseConfig, overrides).
• Documents how to retain the default projects when adding projects because Playwright replaces arrays during config merging.
• Adds a regression test that verifies both Playwright templates are included and connected.

This keeps Grafana-owned defaults updateable without overwriting plugin-specific Playwright customizations. It applies to newly scaffolded plugins; it intentionally does not rewrite existing root configs because those files may contain user changes.

This is a clean reimplementation of the stale-closed #1495 and incorporates its maintainer review feedback, including the requested projects override documentation.

Which issue(s) this PR fixes:

Fixes #1366

Tests:

• npm exec nx run @grafana/create-plugin:test -- --run --no-cache (272 passed)
• npm exec nx run @grafana/create-plugin:lint -- --no-cache
• NX_SKIP_NX_CACHE=true npm exec nx run @grafana/create-plugin:typecheck
• NX_SKIP_NX_CACHE=true npm exec nx run @grafana/create-plugin:build
• npm run docs:test:ci (12 passed)
• npm run docs:build
• Generated a panel plugin with the built CLI, installed its dependencies, ran npm run typecheck, and ran npm exec playwright test -- --list (4 tests listed across the default auth and chromium projects)

Special notes for your reviewer:

The generated root config retains the commented dotenv hook so users can customize environment loading without editing .config/.

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Commit	Author	Reason	Message
4154f6ec	alectimison-maker	unsigned	feat(create-plugin): make Playwright config extendable

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