feat: enhance transform and evalModule to support async callbacks

[productdevbook]

close: #363

In some setups like Unimport, we need to modify the source to add imports at the beginning of each content. Since sourceTransformer is empty every time the eval mode transform runs, I store and cache it to reuse later.

Example code:

  const jiti = createJiti(test.options.rootDir, {
    fsCache: false,
    moduleCache: false,
    debug: true,
    alias: test.options.alias,
  })


  const test = await jiti.evalModule(
    injectedResult.code,
    {
      filename: path,
      async: true,
      conditions: test.options.conditions,

    },
    async (data, name) => {
      return (await test.unimport!.injectImports(data, name)).code
    },
  )