feat: Add support for syncing sections of code with README

[kvankova]

This PR adds support for syncing only sections of code with README. Sections are depicted in README as

  ```language:path/to/script:section_name
  ```

For example, we have the following README file:

# README

This is a readme.

```python:main.py:A
```

The main.py file contains the following code:

print("Hello, world!")

# code_embedder:A start
print("Embedding successful")
# code_embedder:A end

Once the workflow runs, the code block section will be updated in the README file with the content of the section A from the script located at main.py and pushed to the repository 🚀.

# README

This is a readme.

```python:main.py:A
print("Embedding successful")
```

With any changes to the section A in main.py, the code block section is updated in the README file with the next workflow run.

Closes #9