Add explicit toString()

[saschanaz]

Fixes the lib.dom.d.ts part of microsoft/TypeScript#30225 (comment).

This provides autocomplete for types with their own toString() implementation (as toString() inherited from Object is shadowed by default), and also helps third-party tools that depends on TS type library.

const matrix = new DOMMatrix();
matrix.to // should now autocomplete `toString`

cc @domenic

(This restores cpx removed in #753, as it broke npm run baseline-accept.)