Ark: Work with roxygen2 to be able to mark completions as `Deprecated`

i.e. using this from the LSP Spec https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#completionItemTag

/**
 * Completion item tags are extra annotations that tweak the rendering of a
 * completion item.
 *
 * @since 3.15.0
 */
export namespace [CompletionItemTag](https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#completionItemTag) {
	/**
	 * Render a completion as obsolete, usually using a strike-out.
	 */
	export const Deprecated = 1;
}

Ive seen this used in Typescript completions before. Its pretty cool because the completion shows up grey with a strike out, but can still show you the documentation which often tells you what to use instead.

We'd probably add a new @lifecycle tag to roxygen2? Or maybe use the declare() syntax somehow. The exact way to add the metadata is a little uncertain. A @lifecycle tag might be hard because it would apply to all functions on that topic page, which might not be right.

Start by reviewing the LSP CompletionItemTag specification and roxygen2's available metadata mechanisms; the issue names no files or tests. Determine how deprecated completion metadata can be attached without affecting unrelated functions, then verify that deprecated items are struck through while their documentation remains available.