feat: add Deno language server support

[shiersa]

Summary

Adds a Deno language server backed by the Deno CLI's built-in deno lsp, serving TypeScript/JavaScript in Deno projects. Unlike the plain typescript-language-server it understands Deno module resolution (npm: / jsr: / https: imports) and the Deno.* global namespace.

It is experimental and must be selected explicitly via language_servers: [deno] — it overlaps the TypeScript server on file extensions (.ts/.tsx/.js/.jsx/…), so it is not auto-detected. Requires the deno CLI on PATH (it bundles the language server).

This is a fresh implementation against the current DependencyProvider architecture, following .serena/memories/adding_new_language_support_guide.md. (There is an older, stalled PR #1048 from before that refactor; this does not build on it and deliberately avoids the auto-detect / replace-TypeScript behaviour that overlapping extensions can't disambiguate.)

Changes

• src/solidlsp/language_servers/deno_language_server.py — the language server (deno lsp, single-path dependency provider, TS-family language ids)
• src/solidlsp/ls_config.py — DENO enum, file-extension matcher, factory wiring, experimental flag
• test/solidlsp/deno/test_deno_basic.py + test/resources/repos/deno/test_repo/ — tests + a minimal Deno repo
• pyproject.toml — deno pytest marker
• .github/workflows/pytest.yml — the deno marker joins the other-langs batch, which now installs Deno via denoland/setup-deno
• README.md, docs/01-about/020_programming-languages.md, src/serena/resources/project.template.yml, CHANGELOG.md — docs

Testing

Tests run in CI in the other-langs batch on all three OSes: language-server startup, cross-file definition, cross-file references, document symbols, full symbol tree, and two hover tests — one on a local function signature, one on Deno.cwd(), i.e. the Deno.* namespace resolution that is the reason to use this server over the plain TypeScript one. Wherever the deno CLI is absent the tests skip through the central conftest guard, following the existing pattern for toolchain-gated language servers.

Also verified locally against deno 2.9.0 (7 passed), along with ruff lint/format and ty type checks.

Checklist

• This PR follows the guidelines in CONTRIBUTING.md regarding the scope of PRs (isolated addition of a new language server).
• For changes that add features, I have added an entry to CHANGELOG.md.