Add TypeScript support

[SrAnthony]

Summary

• Converts all source files from JavaScript to TypeScript with full type annotations
• Exports RequestOptions, ResponseKind, and Interceptor types for consumers
• Generates .d.ts declaration files via @rollup/plugin-typescript
• Adds "types" field to package.json pointing to dist/index.d.ts

Changes

• Source: Renamed all src/*.js → src/*.ts with strict type annotations
• Build: Added @rollup/plugin-typescript to Rollup pipeline; declarations emitted to dist/
• Config: Added tsconfig.json, src/global.d.ts (Window.Turbo augmentation)
• Tests: Unchanged (JS), work via @babel/preset-typescript
• CI: Added typecheck and build steps, bumped actions to v4
• Linting: Removed standard (no TS support); TypeScript compiler provides strictness

Test plan

• yarn typecheck passes with zero errors
• yarn test — all 69 existing tests pass without modification
• yarn build — produces dist/requestjs.js + .d.ts declaration files
• Verify dist/index.d.ts exports all public types correctly

[marcelolx]

At the moment, there is no intention to convert this codebase to TypeScript — For the same reasons Turbo is plain javascript as well.

[richardkmiller]

@SrAnthony Maybe https://definitelytyped.org/ ? I would use it that way if you end up going down that path.