feat(js): implement fetch api for js runtime

[shouya]

This PR is to implement a simple fetch api for the JS runtime. It enables the user to request data from servers within any JavaScript code.

Here's the API:

type RequestParam = {
  method: "GET" | "POST" | "PUT" | "DELETE",
  headers: { [key: string]: string },
  body: string?
}

fetch(url: string, params?: RequestParam): Promise<Response>;

class Response {
  status: number,
  headers: { [key: string]: string },
  body: string | null

  json(): Object
}

Here's a sample endpoint that returns de-clickbait-ed youtube feed using DeArrow service:

  - path: /kurtzgesagt-dearrow.xml
    source: https://www.youtube.com/feeds/videos.xml?channel_id=UCsXVk37bltHxD1rDPwtNM8Q
    filters:
      - modify_post: |
          const video_id = post.id.split(':').pop();
          let resp = await fetch(`https://sponsor.ajay.app/api/branding?videoID=${video_id}`);
          if (resp.status == 200) {
            let [title] = resp.json().titles;
            post.title.value = title.title;
          }
          let thumbnail = post.extensions.media.group.filter(g => g.name == 'media:group')[0].children.thumbnail[0];
          thumbnail.attrs.url = `https://dearrow-thumb.ajay.app/api/v1/getThumbnail?videoID=${video_id}`;

(Fixes #41)

A few more subtle changes:

• errors are handled and displayed more nicely: instead of throwing the error's internal struct at the user, now we display the error message instead
• js syntax errors will now show the code context
• exceptions raised within modify_post are now reported and ignored, so they won't break the feed
• both modify_post and modify_feed can be defined as async functions
• the modify_post function are now called concurrently for each post, this is especially useful when the function contains long-time async operations.