feat: export TestFunctionCtx type

[KuznetsovRoman]

After this PR, user would be able to use clean module augmentation. Example:

import type { TestFunctionCtx as TestplaneTestFunctionCtx } from "testplane";

declare module 'testplane' {
    export interface TestFunctionCtx extends TestplaneTestFunctionCtx {
        myFunc(var1: string, var2: number): Promise<void>;
    }
}

Also he would be able to pass test function context to other functions and type them properly:
#939