This commit introduces a highly requested `trailingSlash` configuration option to the Angular Router, allowing developers to control how trailing slashes are handled in their applications. The options are:
- 'always': Enforces a trailing slash on all URLs.
- 'never': Removes trailing slashes from all URLs (default).
- 'preserve': Respects the presence or absence of a trailing slash as defined in the UrlTree.

Additionally, this commit updates the `defaultUrlMatcher` to optionally consume a single trailing empty path segment if the route otherwise matches. This ensures that routes with `pathMatch: 'full'` can correctly match URLs with trailing slashes (e.g., `/a/` matching `{path: 'a', pathMatch: 'full'}`), resolving a long-standing inconsistency where such routes would fail despite `pathMatch: 'prefix'` variants (with empty children) succeeding.

**Alternatives Considered & Design Rationale:**

*  **`UrlTree.hasTrailingSlash` Property:**
    We initially explored adding a explicit boolean property to `UrlTree`.
    However, this would have required a significant public API change and breaking updates to custom `UrlSerializer` implementations to ensure the state was preserved. It also complicated the mental model of the `UrlTree` as a direct representation of the URL structure. We settled on representing trailing slashes as a trailing empty `UrlSegment`, which leverages the existing structure and serialization logic without API expansion.

    **Why we chose the current approach:**
    We decided on a "Permissive Matcher, Strict Serializer/Location" pattern.
    -   The **Matcher** is permissive: it allows consuming a trailing empty segment if it's the *only* thing left, treating it effectively as optional for `pathMatch: 'full'`. This aligns logic with how `pathMatch: 'prefix'` matches empty children.
    -   The **Serializer/Location** are strict: The `trailingSlash` configuration is verified at the boundaries (serialization and location normalization). If `trailingSlash: 'never'` is set, the slash is stripped *before* matching ever occurs. If matching sees a slash, it implies the configuration allowed it (or we are in a permissive mode), so the matcher should handle it gracefully.

fixes angular#16051