chore: migrate unhead to v3, typescript to v6 (#2602)

* chore: update deps

* chore: migrate unhead to v3, typescript to v6

* test: add tsnapi snapshots for @slidev/types and @slidev/parser, gated by tsdown-stale-guard

* fix(types): skip node_modules bundling in dts to avoid CJS d.ts errors

tsdown 0.22 + rolldown-plugin-dts can't bundle transitive CJS d.ts
files (postcss, esbuild, webpack-virtual-modules). Treat all
node_modules as external so the bundled dts references them as
imports instead of inlining.

* fix(ci): silence TS 6 deprecation for downlevelIteration

Cypress 14.5.4's webpack-preprocessor injects `downlevelIteration: true`
into its internal ts-loader config. TS 6 errors on it as deprecated and
blocks spec compilation. Set `ignoreDeprecations: "6.0"` at the root so
the cypress tsconfig inherits it and the deprecation downgrades to a
silent acceptance until TS 7.

* chore: upgrade cypress to v15

v15 officially supports TypeScript 6. None of its breaking changes
(SelectorPlayground rename, cy.stub 3-arg signature, cy.exec exitCode,
Firefox CDP, Node 18/23 drop) touch our specs, which only use
cy.visit/get/wait/url/type.

* fix(docs): silence TS 6 deprecation for baseUrl in twoslash

Twoslash's virtual TS env sets baseUrl in its default compiler options,
and TS 6 errors on it. Pass `ignoreDeprecations: '6.0'` through
twoslashOptions.compilerOptions so the doc build's snippet compilation
stops failing. Also remove the unused baseUrl from docs/tsconfig.json.

* chore(docs): regenerate auto-generated components.d.ts

* fix(slidev): silence TS 6 deprecation in CLI twoslash transformer

The vitepress-twoslash transformer Slidev installs for code blocks
hits the same TS5101 baseUrl/moduleResolution deprecation as the docs
build did. Pass `ignoreDeprecations: '6.0'` so the demo's starter
slides (which feed Netlify's deploy preview) compile under TS 6.