feat: inline modules

[corvusrabus]

Draft for non-nested inline modules according to the syntax mainly agreed upon in #2442.

Syntax:

# foo is an inline module!
foo::
  hello:
    @echo "Hello from inside the inline foo module"

Contributes to #2252.

I think there's still a couple of open questions that need to be clarified before something like this PR can be merged

• Should the entire module code depending on filepaths be refactored to accommodate for inline modules that don't directly correspond to a file.
• what should module_directory and module_directory do when used inside an inline module? Should probably just fail?
• Probably more

[musjj]

Hey, are you still working on this?