fix: playground not dimming unused `using` statements (#10815)

The playground wasn't fading out unused `using` declarations the way the
VS Code extension does, even though the language server was already
detecting them correctly.

**Root cause:** The language server running in the playground (for
completions, hover, semantic tokens, etc.) was sending LSP diagnostics
with `DiagnosticTag.Unnecessary` for unused `using` statements, but
`sendDiagnostics` in `services.ts` was a no-op — those tags were never
forwarded to Monaco.

## Changes

- **`packages/playground/src/services.ts`** — Implement
`sendDiagnostics` to filter diagnostics that carry visual tags and apply
them as Monaco `Hint`-severity markers under a separate `"lsp-tags"`
owner. Using `Hint` avoids duplicating the error/warning squiggles
already managed by the playground's own compilation pass.
`DiagnosticTag` values from LSP map directly to `MarkerTag` values in
Monaco (`Unnecessary = 1`, `Deprecated = 2`).

- **`packages/playground/src/react/playground.tsx`** — Fix a
pre-existing bug where `CompletionItemTag.Deprecated` (value `1`, from
`vscode-languageserver`) was used as a Monaco marker tag. This
coincidentally equals `MarkerTag.Unnecessary` (faded text) rather than
the intended `MarkerTag.Deprecated` (strikethrough). Replaced with
`MarkerTag.Deprecated` and removed the `vscode-languageserver` import.

<img width="469" height="200" alt="image"
src="https://github.com/user-attachments/assets/587567b3-6c1d-4243-8257-93fb6a6fc977"
/>

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: timotheeguerin <1031227+timotheeguerin@users.noreply.github.com>
Co-authored-by: Timothee Guerin <tiguerin@microsoft.com>