fix: handle function overloads in .d.ts bundling

[hugs7]

fix: handle function overloads in .d.ts bundling

Fixes #209

Problem

When isolatedDeclarations is not enabled, rolldown-plugin-dts uses tsc to generate .d.ts files. For function overloads, tsc correctly emits multiple export declare function declarations with the same name:

export declare function useConfig(): Config;
export declare function useConfig<T>(selector: (config: Config) => T): T;

During the transform phase, the fake-js plugin converts each declaration into a var useConfig = [id, deps, children] assignment. With overloads, this produces two export var useConfig statements, which Rolldown's parser rejects as duplicate exports.

Solution

The fake-js plugin now detects function overload signatures during transform and merges them into the primary declaration:

• Transform phase: When a TSDeclareFunction is encountered whose binding name has already been processed, the overload's declaration, deps, params, and children are stored on the existing DeclarationInfo rather than emitting a new runtime variable. The overload's deps/params/children are appended to the primary's arrays so they go through Rolldown's identifier renaming pipeline.

• RenderChunk phase: When restoring a declaration that has overloads, all overload signatures are emitted alongside the primary declaration in original source order, with proper identifier renaming applied via the merged arrays.

Test

Added a function-overloads test fixture that verifies both overload signatures appear in the bundled output:

interface Config {
  name: string
  version: number
}

export function useConfig(): Config
export function useConfig<T>(selector: (config: Config) => T): T
export function useConfig<T>(selector?: (config: Config) => T) {
  const config: Config = { name: 'app', version: 1 }
  return selector ? selector(config) : config
}