[breadcrumbs] Task 1: scaffold vaadin-breadcrumbs-flow-parent module with feature-flag plumbing

[web-padawan]

Implementation task 1 of the Breadcrumbs Flow component (spec-driven development pipeline).

What to do:

• Create the vaadin-breadcrumbs-flow-parent module with three sub-modules (vaadin-breadcrumbs-flow, vaadin-breadcrumbs-flow-integration-tests, vaadin-breadcrumbs-testbench) and their pom.xml wiring
• Add empty class shells: Breadcrumbs (@Tag/@NpmPackage/@JsModule, extends Component implements HasSize, HasStyle, HasAriaLabel, HasComponentsOfType<BreadcrumbsItem>) and BreadcrumbsItem (implements HasText, HasEnabled, HasPrefix)
• Add the experimental feature-flag plumbing: BreadcrumbsFeatureFlagProvider, ExperimentalFeatureException, the META-INF/services ServiceLoader registration, and an onAttach feature-flag check
• Enable the flag in the IT module via src/main/resources/vaadin-featureflags.properties
• Add empty TestBench element shells BreadcrumbsElement / BreadcrumbsItemElement
• Register the new module in the root pom.xml
• Smoke tests: new Breadcrumbs() / new BreadcrumbsItem("Home") instantiate, the component round-trips through Java serialization, and the feature-flag check throws ExperimentalFeatureException when disabled (and not when enabled)

Spec references:

• flow-tasks.md — Task 1
• flow-spec.md

Part of vaadin/web-components#7960