Add regex control over separators in character text splitter (#7933)

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#7854

Added the ability to use the `separator` ase a regex or a simple
character.
Fixed a bug where `start_index` was incorrectly counting from -1.

Who can review?
@eyurtsev
@hwchase17 
@mmz-001

docs/snippets/modules/data_connection/document_transformers/text_splitters/character_text_splitter.mdx

@@ -12,6 +12,7 @@ text_splitter = CharacterTextSplitter(
     chunk_size = 1000,
     chunk_overlap  = 200,
     length_function = len,
+    is_separator_regex = False,
 )
 ```
 

docs/snippets/modules/data_connection/document_transformers/text_splitters/recursive_text_splitter.mdx

@@ -16,6 +16,7 @@ text_splitter = RecursiveCharacterTextSplitter(
     chunk_size = 100,
     chunk_overlap  = 20,
     length_function = len,
+    is_separator_regex = False,
 )
 ```