Consolidate XAML markup-extension metadata catalogs

Is your feature request related to a problem? Please describe.

The XAML language server currently maintains related markup-extension metadata in three places: completion labels/details, curated hover descriptions, and XAML-language directive quick info. The payloads serve different UI surfaces, but overlapping names and descriptions can drift over time.

Describe the solution you'd like

Introduce a shared markup-extension descriptor catalog that exposes the common identity and semantics for x:Bind, Binding, StaticResource, ThemeResource, TemplateBinding, RelativeSource, x:Static, x:Type, and x:Null. Completion and hover providers should project their surface-specific detail/markdown from that catalog. Keep attribute-only XAML directives separate because their resolution semantics differ.

Additional context

Identified during PR #50's hover-performance review. This refactor is intentionally separate from the latency/correctness fix to avoid expanding its risk and scope.

Start by locating the completion-label/detail metadata, curated hover descriptions, and XAML-language directive quick-info implementations, then compare their overlapping markup-extension entries. Done means a shared descriptor catalog provides the common identity and semantics for the listed extensions, completion and hover project their own surfaces from it, and attribute-only directives remain separate.