feat: core.send_code_block

[frere-jacques]

Editors like vscode or pycharm or tools like jupyter notebooks let you send code blocks devided by a delimiter, eg. in python '#%%' or '# %%' to the repl.

The plugin jupytext is also to mention here, as it creates a .py representation of jupyter notebooks, in which in case of the py:percent style, the code blocks are delimited with '# %%'.

I wrote this feature which lets you define multiple code_deviders in the repl_definition and let you send the lines between those to the repl. Optionally one can choose to move the cursor to the next block in order to iterate easily through the file.

I also updated the doc and the readme to reflect for the changes and included two commands to the core.named_maps.

I hope you like, what I did here, and see the comfortable workflow provided by it.

I asked here https://github.com/Vigemus/iron.nvim/discussions/386#discussion-7045785, whether this would be possible, but then had fun, implementing it myself.