chore: update core services and module export

[louis-jan]

Problem

Since we renamed @janhq/plugin-core to @janhq/core, its not so reasonable to

import { core } from "@janhq/core" ;
core.downloadFile("");

// OR

import * as core from "@janhq/core"
core.core.downloadFile("")

Solution

Core module should export all functions so:

import { downloadFile } from "@janhq/core" ;
downloadFile("");

// OR

import * as core from "@janhq/core"
core.downloadFile("")