rfctr(chunking): extract general-purpose objects to base (#2281)

Many of the classes defined in `unstructured.chunking.title` are
applicable to any chunking strategy and will shortly be used for the
"by-character" chunking strategy as well.

Move these and their tests to `unstructured.chunking.base`.

Along the way, rename `TextPreChunkBuilder` to `PreChunkBuilder` because
it will be generalized in a subsequent PR to also take `Table` elements
such that inter-pre-chunk overlap can be implemented.

Otherwise, no logic changes, just moves.

CHANGELOG.md

@@ -1,4 +1,4 @@
-## 0.11.5-dev1
+## 0.11.5-dev2
 
 ### Enhancements