No, I am not talking about editors such as EditPlus and Notepad++ that can open multiple distinct text files. I am looking for a tool that can create a single text file having multiple sheets in it. Idea is simple -- to maintain my many related text files in a single workbook. Is something available out there to achieve this?
There are a few options similar to but not necessarily an exact match to what you describe.
There used to be a few tools that would use a single executable with a single text file but not sure if any of them still exist.
You also have the Jupyter Notebook which uses a version of Python so again not really a single file and also not really a text file (well it sort of is) since it uses a JSON format.
If you were to relax your "rules" a little, there are plenty of tools that use folders and text files (generally a simple text markup format such as MarkDown). Let me know in the comments if those are interesting and I will try to dig some out as examples.
Not sure whether it's possible at all: Having a text format supporting different sheets would need some markup. When talking about xls-like format, there will be formula and other things. My guess is, no chance to do it with a simple, stream lined format.
But: The OpenDocument format as supported e.g. by LibreOffice is supporting a version where everything is stored inside an unzipped XML file. The filetype is called
OpenDocument Flat XML Table. Beside of some format informations, it should be possible to edit with many OpenDocument compatible tools as well as your favorite text editor.