IsolatedDeclaration: support rewriteImportExtension option

[Dunqing]

The rewriteImportExtensions option now works as configured/desired for source files; however, it doesnt apply to the generated declarations file.

Input

// src/example.ts
import { Provider, type MODEL } from './model.ts';

export async function foobar(): string { ... }

Output: rewriteImportExtensions: 'rewrite'

// out/example.js ✅
import { Provider } from './model.js';

export async function foobar() {
  // ...
}

// out/example.d.ts ❌
import type { MODEL } from "./model.ts"; // <<< path does not exist

export declare function foobar(): string;
export {};

Output: rewriteImportExtensions: 'remove'

// out/example.js ✅
import { Provider } from './model';

export async function foobar() {
  // ...
}

// out/example.d.ts ❌
import type { MODEL } from "./model.ts"; // <<< path does not exist

export declare function foobar(): string;
export {};

---

In both scenarios, the example.d.ts file's import paths are unchanged. This is/will always be invalid since the source .ts file will not exist post-transform.

While the generated d.ts content does match tsc output, it only works when the dts is placed as sibling files to the source ts files. And for a npm context, the source *.ts files would have to be published alongside the *.d.ts files to work, which honestly makes the *.d.ts files redundant/irrelevant, since all types/enums/func definitions are repeated.

Actually, this could create scenarios where more complex generics are broken because two copies of the complex type are loaded (1 from .ts and 1 from d.ts) and are sufficiently complex that tsc treats them as separate types, breaking narrowing, inference, etc.

Originally posted by @lukeed in #5395