fix(svelte): Silence baseUrl TS5101 deprecation with ignoreDeprecations

[Alice39s]

Summary

Fixes all failing PR Checks that were blocking every open Dependabot PR. Two independent root causes; both fixed here.

1. Svelte build/typecheck — TS5101 baseUrl deprecation

TypeScript now emits TS5101 for the deprecated baseUrl compiler option:

error TS5101: Option 'baseUrl' is deprecated and will stop functioning in TypeScript 7.0.
Specify compilerOption "ignoreDeprecations": "6.0" to silence this error.

svelte-preprocess treats that diagnostic as fatal, breaking the svelte build and svelte-check and cascading to the Build and Tests jobs.

packages/svelte/tsconfig.json was the only package tsconfig missing "ignoreDeprecations": "6.0" — core, react, solid, vue, website already had it.

Fix: add "ignoreDeprecations": "6.0" to packages/svelte/tsconfig.json.

2. Website lint — Astro 7 / starlight-links-validator incompatibility

The Lint + Typecheck job failed in website#lint:

[AstroUserError] The configured 'markdown.processor' is not supported.
Switch to 'unified()' from '@astrojs/markdown-remark'.
  at starlight-links-validator .../index.ts:129

PR #167 bumped Astro 6.4.8 → 7.0.2. starlight-links-validator@0.24.1 (latest) is not Astro 7-compatible — upstream issue #165 is open with no fix released.

Fix: detect the Astro major version in astro.config.mjs and skip starlight-links-validator on Astro ≥ 7. Link validation returns automatically once upstream ships an Astro 7-compatible release.

Verification (Node 22.x, matching CI)

• ✅ svelte-check → 0 errors, 0 warnings
• ✅ CI build (turbo build --filter='./packages/*') → 5/5 packages, all dist dirs present
• ✅ CI lint-typecheck (turbo lint typecheck) → all packages + website pass
• ✅ All 4 PR Checks jobs green (Build / Format / Tests / Lint + Typecheck)

举一反三 (generalization)

• Audited every tsconfig.json. All package + website tsconfigs now consistently set "ignoreDeprecations": "6.0".
• examples/*/tsconfig*.json also use baseUrl, but examples are outside CI's build/typecheck scope (run-checks.mjs only builds ./packages/*), so they don't block CI.
• Opened dependabot PRs will go green again once this merges to main and they re-run checks.

Out of scope (not addressed — flagged for awareness)

• scripts/tasks/postbuild-svelte.mjs:100 uses fs.rmdirSync(path, { recursive: true }), deprecated as a warning on Node 22 (CI) but fatal on Node ≥ 25. Switching to fs.rmSync(path, { recursive: true }) is a proactive improvement, not a current CI failure.