Add support for ID-based links

[wgslr]

Implement functionality inspired by Zettlr -- the ability to store ids in files and link to them with wiki-style links. For example:

file1.md

# This is a file having an id
This is its id: 20200522225822

file2.md

# This is a file linking to another
See the other file: [[20200522225822]]

The file id is the first match of a (configurable) regex in its contents.

[tchayen]

I am aware it is WIP, but I thought some things could be improved.

[tchayen]

Last batch of requests.

BTW, please remove package-lock.json. The extension will be published with yarn-downloaded dependencies anyway, so it only creates a confusion with two of those files. Especially that I won't be using it so the contents will quickly get outdated.