Wikilink navigation in markdown preview panel (#521)

* `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

.vscode/launch.json

@@ -4,12 +4,21 @@
 // For more information, visit: https://go.microsoft.com/fwlink/?linkid=830387
 {
 	"version": "0.2.0",
+  "inputs": [
+    {
+      "id": "packageName",
+      "type": "pickString",
+      "description": "Select the package in which this test is located",
+      "options": ["foam-core", "foam-vscode"],
+      "default": "foam-core"
+    }
+  ],
 	"configurations": [
 		{
 			"type": "node",
 			"name": "vscode-jest-tests",
 			"request": "launch",
-			"runtimeArgs": ["workspace", "foam-core", "run", "test"], // ${yarnWorkspaceName} is what we're missing
+			"runtimeArgs": ["workspace", "${input:packageName}", "run", "test"], // ${yarnWorkspaceName} is what we're missing
 			"args": [
 					"--runInBand"
 			],

.vscode/settings.json

@@ -22,5 +22,10 @@
   "prettier.requireConfig": true,
   "editor.formatOnSave": true,
   "editor.tabSize": 2,
-  "jest.debugCodeLens.showWhenTestStateIn": ["fail", "unknown", "pass"]
+  "jest.debugCodeLens.showWhenTestStateIn": [
+    "fail",
+    "unknown",
+    "pass"
+  ],
+  "gitdoc.enabled": false
 }