feat: Add tsdown build setup with TypeScript support

[akash-joshi]

Hi folks, this PR adds tsdown build tooling with WebAssembly support, enabling TypeScript development for harfbuzzjs. This addresses the discussion in #99 about modernising the TypeScript setup.

This was based on @lianghai 's comment on the Svelte London chat. I wanted to create a rough draft of what a tsdown-based setup could look like.

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AI Generated Summary Below

• Add tsdown configuration with rolldown-plugin-wasm for WASM support
• Add TypeScript configuration and type definitions for hbjs.js and hb.js
• Add TypeScript entry point (src/index.ts) that re-exports types
• Update package.json with dual ESM/CJS exports and build scripts
• Bundle outputs to dist/ with generated .d.ts declaration files

Changes

New files

• tsdown.config.ts - Build config with WASM plugin
• tsconfig.json - TypeScript compiler configuration
• src/index.ts - TypeScript entry point
• src/types.d.ts - Type definitions (HBHandle, HBFont, HBBuffer, etc.)
• hbjs.d.ts - Sidecar type declarations for hbjs.js
• hb.d.ts - Sidecar type declarations for hb.js

Modified files

• package.json - Added build scripts, exports field, and dev dependencies
• .gitignore - Added dist/

Build output

dist/
├── index.cjs      (58.5 kB) - CommonJS bundle
├── index.mjs      (58.7 kB) - ESM bundle
├── index.d.cts    - CJS type declarations
├── index.d.mts    - ESM type declarations
└── hb.wasm        - Copied WASM binary

Test plan

• TypeScript compiles without errors (npx tsc --noEmit)
• Build succeeds (npm run build:ts)
• All 68 tests pass (npx mocha test/index.js)