Rename TextChunker from SemanticTextPartitioner - #683

### Motivation and Context
To match changes in #450 

### Description
This pull request renames the SemanticTextPartitioner class to
TextChunker, and updates all references to this class in the code and
documentation. The TextChunker class is responsible for splitting text
into smaller chunks based on tokens and paragraphs, which is useful for
processing large texts or code files. The new name TextChunker reflects
the functionality of the class more clearly and avoids confusion with
the SemanticPartitioner class, which is a different concept that splits
text into semantic segments based on embeddings.

Details:
- Rename SemanticTextPartitioner.cs to TextChunker.cs and update the
class name and namespace accordingly. The TextChunker class is now in
the Text namespace, which is more consistent with its purpose and the
other classes in the namespace.
- Update all references to SemanticTextPartitioner in the code and
documentation to use TextChunker instead. This includes the
ConversationSummarySkill and the DocumentImportController classes, which
use the TextChunker class to split text into sentences and paragraphs
for summarization and import.
- Update the README.md file for the GitHub Repo Q&A Bot sample to
explain how the TextChunker class works with memory and embeddings.
- Make some minor formatting changes to the TextChunker class, such as
adding line breaks and spaces for readability, and removing an unused
using directive.

### Motivation and Context
To match changes in microsoft#450 

### Description
This pull request renames the SemanticTextPartitioner class to
TextChunker, and updates all references to this class in the code and
documentation. The TextChunker class is responsible for splitting text
into smaller chunks based on tokens and paragraphs, which is useful for
processing large texts or code files. The new name TextChunker reflects
the functionality of the class more clearly and avoids confusion with
the SemanticPartitioner class, which is a different concept that splits
text into semantic segments based on embeddings.

Details:
- Rename SemanticTextPartitioner.cs to TextChunker.cs and update the
class name and namespace accordingly. The TextChunker class is now in
the Text namespace, which is more consistent with its purpose and the
other classes in the namespace.
- Update all references to SemanticTextPartitioner in the code and
documentation to use TextChunker instead. This includes the
ConversationSummarySkill and the DocumentImportController classes, which
use the TextChunker class to split text into sentences and paragraphs
for summarization and import.
- Update the README.md file for the GitHub Repo Q&A Bot sample to
explain how the TextChunker class works with memory and embeddings.
- Make some minor formatting changes to the TextChunker class, such as
adding line breaks and spaces for readability, and removing an unused
using directive.

### Motivation and Context
To match changes in microsoft#450 

### Description
This pull request renames the SemanticTextPartitioner class to
TextChunker, and updates all references to this class in the code and
documentation. The TextChunker class is responsible for splitting text
into smaller chunks based on tokens and paragraphs, which is useful for
processing large texts or code files. The new name TextChunker reflects
the functionality of the class more clearly and avoids confusion with
the SemanticPartitioner class, which is a different concept that splits
text into semantic segments based on embeddings.

Details:
- Rename SemanticTextPartitioner.cs to TextChunker.cs and update the
class name and namespace accordingly. The TextChunker class is now in
the Text namespace, which is more consistent with its purpose and the
other classes in the namespace.
- Update all references to SemanticTextPartitioner in the code and
documentation to use TextChunker instead. This includes the
ConversationSummarySkill and the DocumentImportController classes, which
use the TextChunker class to split text into sentences and paragraphs
for summarization and import.
- Update the README.md file for the GitHub Repo Q&A Bot sample to
explain how the TextChunker class works with memory and embeddings.
- Make some minor formatting changes to the TextChunker class, such as
adding line breaks and spaces for readability, and removing an unused
using directive.