feat(lang): format and lint Astro like Svelte

[aimuzov]

Summary

Astro gets the same treatment Svelte already had. Upstream `lang.astro` sets only `prettier` as the formatter.

• Formatting: new override `overrides/lazyvim/lang-astro.lua` — with `lang.astro` and `formatting.prettier`, `.astro` files run `prettier` then `stylelint`.
• Diagnostics: with `linting.eslint`, `stylelint_lsp` attaches to `astro`; the `linting.stylelint` extra validates `astro`.
• Shared formatter: the stylelint conform formatter (project `node_modules/.bin/stylelint`, `--custom-syntax postcss-html` for markup files) moves from `lang-svelte.lua` to `util/stylelint.lua` and serves both languages.
• DAP: `dap.vscode-js` configurations cover `astro`.
• Docs: override counts 39 → 40 (LazyVim 9 → 10) across README, ARCHITECTURE, EXTRAS, FAQ.
• `.gitignore`: the stale chezmoi path becomes `.luarc.json` (local LuaLS settings).

Also carries `fix(formatting): pick oxfmt only by its own config` from the same branch: the oxlint config alone no longer switches a project from prettier to oxfmt.

Test plan

• `npm run format:check`, `npm run lint`
• Real config, headless, on lazyvimx/site: `home.astro` → conform runs `prettier, stylelint` (project stylelint), formatting succeeds; LSP clients include `astro` and `stylelint_lsp`
• Same check on `demo-player.svelte`: `prettier_svelte, stylelint`, unchanged behavior after the refactor