What is this for?

To showpreview orsolution webview, a problem node must be provided.

There are two steps to retrieve the problem node from the text document.

1. Get meta info from text document.
2. Get problem node with the id in the meta info.

This PR works on the first step, providing the meta info needed (if we use file path as show keyword).

This is a way to reuse the meta info parsing logic ofleetcode-cli, since the logic is so coupled with cli utils that it's hard to extract it tovscode-leetcode.

Another way is to add an option likeleetcode meta <filename>, but it will introduce more code.