feat: support dynamic imports with addRelativeDeclarationExtensions

[cjpearson]

#267

This patch extends the addRelativeDeclarationExtensions functionality to also add the extension when import() is used.

Before:

export * from "./other.js";
export type { Other } from "./other.js";
export declare function wonder(twinkle: import("./other").Other): string;

After:

export * from "./other.js";
export type { Other } from "./other.js";
export declare function wonder(twinkle: import("./other.js").Other): string;

[cjpearson]

A simpler option might be to take a substring, but I wasn't sure if there might a case with an actual dynamic import where the expression is not a string literal but it still passes the RELATIVE_RE test.