build: type the ./css subpath export (drop the @ts-expect-error)

[ryankiley]

Found while integrating tweakit into a Nuxt + vue-tsc app.

Problem

Importing the stylesheet via the subpath export —

import "tweakit/css";

— raised TS2307: Cannot find module 'tweakit/css'. A bare package-subpath import isn't matched by a global *.css ambient, and the ./css export had no types condition, so consumers needed a @ts-expect-error.

Fix (packaging only — source unchanged)

• The build emits a tiny side-effect dist/tweaks.css.d.ts (export {};).
• The ./css export gains a types condition pointing at it; files ships it (109 B).

Verify

• import "tweakit/css" now resolves cleanly under --moduleResolution bundler (what Nuxt/vue-tsc use) — no @ts-expect-error.
• npm test 28/28; declaration present in the published tarball.

Note: the only other DX nit encountered — building a schema object and passing it to tweaks() needing an as Schema cast — turned out to be consumer-side: typing the builder with the already-exported SchemaObject / Schema removes the cast, so no kit change is warranted there.