Add BluditGithubImport class for GitHub article import functionality

- Implemented methods for slug validation, path validation, scalar parsing, and document processing.
- Added link and image handling with safety checks for content.
- Introduced blob verification for GitHub responses and asset type validation.
- Created unit tests for the import functionality, covering various edge cases and ensuring robust error handling.
- Included tests for metadata preservation, asset handling, and concurrent edits.

languages/en.json

@@ -1,6 +1,6 @@
 {
-	"plugin-data": {
-		"name": "GitHub Export",
-		"description": "Export Bludit articles to a GitHub repository as Markdown files with YAML frontmatter."
-	}
+  "plugin-data": {
+    "name": "GitHub Sync",
+    "description": "Export Bludit articles to GitHub and preview/import selected articles, metadata, images and attachments back into Bludit."
+  }
 }