Refactor solution webview to reuse markdown engine

[Vigilans]

Introduction

• Extract markdown rendering logic outside of solution webview as MarkdownEngine class.
• Solved Solution webview doesn't highlight python3 language #205, solving [bugfix] Clicking 'Code Now' will open multiple files #216 (comment)

Details

• Add link validator logic, making it support file:// protocal.
• Enhanced indented code block highlight logic.
• Correction of multiple unsupported alias of language(e.g. python3)
• Completion of image url in solution markdown(e.g. /asset/... -> https://discuss.leetcode.com/asset/..., refer to this solution)

[jdneo]

Could you explain more why we need the validator here?

[Vigilans]

This is the routine adopted by built-in markdown exntesion's MarkdownEngine, I think it may enables us to validate local file link, which may be useful in local debugger.

[jdneo]

Ok... Actually still not quite understand... I think it's a worth investigating topic. It's fine to leave it here in this PR...

[Vigilans]

Document from markdown-it:

MarkdownIt#validateLink(url)Boolean

Link validation function. CommonMark allows too much in links. By default we disable javascript:, vbscript:, file: schemas, and almost all data:... schemas except some embedded image types.

You can change this behaviour:

var md = require('markdown-it')();
// enable everything
md.validateLink = function () { return true; }

Since we will deal with local files in WebView, chances are that it will be helpful to enable file: link.

[Vigilans]

I looked into the source code, and found that it was made to prevent XSS attack. Here are some examples:

• When parsing image and link:

    if (res.ok) {
      href = state.md.normalizeLink(res.str);
      if (state.md.validateLink(href)) {
        pos = res.pos;
      } else {
        href = '';
      }
    }

If validateLink does not pass, then href is screened out.

• The same goes on for reference-link or auto link:

  href = state.md.normalizeLink(res.str);
  if (!state.md.validateLink(href)) { return false; }

So, it is indeed necessary to loose the restriction to allow file:// protocol.

[Vigilans]

Now the markdown engine is fully decoupled with webviews with the help of env parameter:

const body: string = this.markdown.render(solution.body, {
    lang: this.solution.lang,
    host: "https://discuss.leetcode.com/",
});

[jdneo]

Ok... Actually still not quite understand... I think it's a worth investigating topic. It's fine to leave it here in this PR...

[jdneo]

Much much better. Just some small change requests.

Thank you!