Fix for issue #85 Extension hangs / freezes VS Code in big folders

[egamma]

This fix makes two changes:

• instead of parsing all files in parallel, it does it in batches by using the async.eachLimit function with a batch size of 100 (see the change in extension.ts).
• it replaces the use of vscode.TextDocument with a cheaper implementation that reads the file directly from disk using the node fs module. I did the change minimally invasive so that the parsers are only minimally affected.

Tested the scalability of the implementation of the chromium code base. The vscode UI was staying responsive and the extension host did not crash 😄. This is the result:

[Extension Host] Summary:
[Extension Host]% 46577 parseable documents found
[Extension Host]% 55894 CSS class definitions found
[Extension Host]% 9822 unique CSS class definitions found
[Extension Host]% 681 failed attempts to parse.

[zignd]

What an impressive improvement 😀, thanks for taking your time. I will test it in a moment.