Use new Text Entry class to track text entries in Intermediate Format

- Context
  - The app maintains all text content in a standard, intermediate format
  - The intermediate format was loaded, passed around as a dictionary
    for easier, faster updates to the intermediate format schema initially
  - The intermediate format is reasonably stable now, given it's usage
    by all 3 text content types currently implemented

- Changes
  - Concretize text entries into `Entries' class instead of using dictionaries
    - Code is updated to load, pass around entries as `Entries' objects
      instead of as dictionaries
    - `text_search' and `text_to_jsonl' methods are annotated with
       type hints for the new `Entries' type
    - Code and Tests referencing entries are updated to use class style
      access patterns instead of the previous dictionary access patterns

  - Move `mark_entries_for_update' method into `TextToJsonl' base class
    - This is a more natural location for the method as it is only
      (to be) used by `text_to_jsonl' classes
    - Avoid circular reference issues on importing `Entries' class

tests/test_text_search.py

@@ -76,7 +76,7 @@ def test_asymmetric_search(content_config: ContentConfig, search_config: SearchC
 
     # Assert
     # Actual_data should contain "Khoj via Emacs" entry
-    search_result = results[0]["entry"]
+    search_result = results[0].entry
     assert "git clone" in search_result