Expose TypeScript type definitions in npm package

Fixes #229. The npm package previously shipped no type definitions,
forcing TypeScript consumers to write their own declare module shims.

Changes:
- Added build:types step (tools/bundle-dts.js) that auto-generates a
  bundled dist/spector.d.ts from the .d.ts files ts-loader already
  emits during the webpack build — zero new dependencies
- Added 'types' field to package.json pointing to dist/spector.d.ts
- Added dist/spector.d.ts to the 'files' array so npm publish includes it
- Added skipLibCheck to tsconfig for third-party type compatibility

Consumers can now use:
  import { Spector, ICapture, Observable } from 'spectorjs';
  import * as SPECTOR from 'spectorjs';

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: sebavan