Move LLM-related components into LLM module (#74)

* Move splitter into indexing module
* Rename post_processing module to parsers
* Migrate LLM-specific composite pipelines into llms module

This change moves the `splitters` module into `indexing` module. The `indexing` module will be created soon, to house `indexing`-related components.

This change renames `post_processing` module into `parsers` module. Post-processing is a generic term which provides very little information. In the future, we will add other extractors into the `parser` module, like Metadata extractor...

This change migrates the composite elements into `llms` module. These elements heavily assume that the internal nodes are llm-specific. As a result, migrating these elements into `llms` module will make them more discoverable, and simplify code base structure.

tests/test_post_processing.py

@@ -1,7 +1,7 @@
 import pytest
 
 from kotaemon.base import Document
-from kotaemon.post_processing.extractor import RegexExtractor
+from kotaemon.parsers import RegexExtractor
 
 
 @pytest.fixture