feature: stitch new files into config

[jerivl]

In line with the goal of bidirectionality for literate programs, it would be nice to be able to create files in the filesystem which then get stitched automatically into some configured default file. I'd be happy to work on a PR that accomplishes the following:

If default-config is defined in entangled.toml and the file is managed by entangled:

• A new unmanaged file gets detected
• The extension for the file is looked up against dict of standard extensions for available languages
• If the extension is valid, then append a new codeblock in the defined markup style to default-config and add the appropriate code fences to the original src file

[jhidding]

I like the idea! Some of my thoughts:

Part of the adoption strategy of Entangled is that you can use it on partial projects. You have some code-base and would like to add a literate module, Entangled ignores the rest of the project, so this feature needs to be disabled by default. On the other hand, I can also see how this feature will make it easier to start expanding a new project. I don't like the name default-config, rather let's call this feature auto_stitch. I would suggest the following API in entangled.toml:

version = "2.5"

[auto_stitch]
source_file = "docs/undocumented.md"  # defaults to None, disabling the feature

[auto_stitch.fences]
# using quarto attributes:
open = "```${language_id}\n${comment}| file: ${target_file}\n"
# alternatively:
#   open = "``` {.${language_id} file=${target_file}}\n"
close = "```\n"

The Language type needs a new field extensions: list[str] = default_factory(list), defaulting to empty extension lists, so that older language definitions still work.

Entangled doesn't know how to format fenced code blocks yet, so you need to teach it. I tend to use Python's string.Template style templates for configurable template strings. Since there may be newlines in these fences, I think it is proper to also have terminating newlines in there (though I can't imagine a case where there wouldn't be one). Notice that we can specify defaults for auto_stitch.fences without enabling the feature, so these defaults can be included in the different config profiles.