* `FoamWorkspace.find` to return `null`  when no reference is provided for relative path

* turning wikilinks into browsable links in markdown preview

* moved preview styles in css file and reorganized code in static folder

Static was previous used only for the dataviz graph. Now we have 2 subdirectories: dataviz for the graph, and preview for the markdown preview.
For now the css style is a bit of an overkill, but sets up the right foundation for further customization down the line.

* chore: explicitly disabling gitdoc extension, removing unnecessary async keyword

* fix: fixed test utility fn (and linter warning)

* test: added tests for preview link generation

* changed launch configuration to support both foam-core and foam-vscode packages