Modernization 3/4: Move build tooling to Rollup

[Ryang-21]

Part of the js-stellar-sdk modernization epic.

Depends on: Task 2 (ESM compliance).

Goal

Replace the Webpack/Babel pipeline with Rollup.

Scope

• Add Rollup configuration producing ESM (dist/esm/), CJS (dist/cjs/), and browser UMD/IIFE (dist/browser/) outputs with sourcemaps.
• Wire up @rollup/plugin-typescript (or rollup-plugin-esbuild) for TS compilation, @rollup/plugin-node-resolve, @rollup/plugin-commonjs for any remaining CJS deps, and @rollup/plugin-terser for minified browser bundles.
• Emit .d.ts rollups via rollup-plugin-dts so published types mirror published JS.
• Update package.json exports, main, module, browser, and types fields to point at Rollup outputs.
• Remove config/webpack.config.browser.js, babel.config.js, and associated dependencies once Rollup replaces them.
• Update scripts/ and package.json build, build:browser, and prepublish scripts.

Acceptance

• yarn build runs Rollup end-to-end and produces the full dist/ tree.
• Bundle size (gzipped) is equal to or smaller than the pre-migration Webpack output.
• npm pack tarball contents match the current published shape; a smoke test install in a scratch Node ESM project and a scratch Vite project succeeds.

Risks

• Rollup tree-shaking behaves differently than Webpack; baseline and compare gzipped sizes before merging.