Support `deprecationMessage` custom keyword

[Suyog241005]

Relates to #85.

Background & Research

In vscode-json-languageservice, deprecationMessage is a custom schema keyword used to indicate that a property is deprecated and provide migration guidance or rationale.
When present in a schema:

1. Hover Tooltips (textDocument/hover): Displays a deprecation notice containing the custom message alongside the title and description. 2. Property Completion (textDocument/completion): Flags deprecated properties with LSP's CompletionItemTag.Deprecated (1), displaying them with strikethrough styling in editor completion lists, and displays the deprecation message in item documentation.
2. Standard Fallback: Gracefully handles standard JSON Schema deprecated: true (e.g. Draft 2019-09 / 2020-12) when no custom message is supplied.

Description

Support VSCode's custom deprecationMessage keyword (as well as markdownDeprecationMessage) in hyperjump-json-language-server:

• Hovering over a deprecated property key or value will render a prominent deprecation notice (⚠️ **Deprecated:** <message>).
• Property completion proposals will flag deprecated properties with CompletionItemTag.Deprecated and attach the message to item documentation.

Proposed Scope

• Update Hover.ts to inspect annotations for unknown#deprecationMessage, unknown#markdownDeprecationMessage, and standard deprecated.
• Update CompletionEvaluationPlugin.ts to record deprecation status in PropertyValueInfo from schema AST.
• Update PropertyCompletion.ts to set tags: [CompletionItemTag.Deprecated] and documentation.
• Add comprehensive unit tests in Hover.test.ts and PropertyCompletion.test.ts.