Spike on FoamWorkspace and DataStore refactor (#593)

* extracted graph from workspace

* refactored datastore

* dataviz to use URI for placeholder detection

* graph uses uris, not resources

* adding placeholder in graph

* link completion to use graph

* API v1 - Resource refactoring (#595)

* aside: tweaked jest extension configuration

* added provider for markdown

* removed file check in URI

the problem is that it makes the URI dependent on the disk, which makes testing harder.
The change was originally introduced to prevent Foam from treating directories ending in .md as markdown files, but the check needs to probably happen somewhere else, e.g. in `FileDataStore.list` - or directories should be expressed with a trailing slash (to check whether that breaks the URI convention)

* Various changes

- `resolveLink` now delegates to providers
- added `read` method in providers and `FoamWorkspace`
- improved `Matcher` API
- updated tests to use workspace with providers
- delegating more to workspace now it can read files (simplifies wiring and exposed API surface)
- provider init returns a promise, so it can be awaited on
- `IDataStore` now has `list` method, to encapsulate all access to FS

* improved windows support in URI and matcher

* improved grouped resources interface

* added readAsMarkdown in provider, useful for tooltip generation with preview in vscode

.vscode/settings.json

@@ -27,5 +27,7 @@
     "unknown",
     "pass"
   ],
-  "gitdoc.enabled": false
+  "gitdoc.enabled": false,
+  "jest.autoEnable": false,
+  "jest.runAllTestsFirst": false
 }