feat: adds support for non-submodule repos

[agoodshort]

Feature request from #3
This PR adds support for non-submodule repos by parsing through the directories to find .git.

The following default extension's configs were added to support this feature:

find_subdirectories = {
    enabled = false,
    depth = 1
}

Next steps

• Add documentation in README.md
• Tested/Reviewed by a user
• Remove print() lines

[agoodshort]

Found an issue while testing, taking note here to refer to later when I have some time to look at it.

When find_subdirectories is enabled and on a neo-tree window I receive the below error:

E5108: Error executing lua: ...odules.nvim/lua/telescope/_extensions/git_submodules.lua:21: attempt to index a nil value
stack traceback:
	...odules.nvim/lua/telescope/_extensions/git_submodules.lua:21: in function 'open_git_tool'
	...odules.nvim/lua/telescope/_extensions/git_submodules.lua:209: in function 'run_replace_or_original'
	...re/nvim/lazy/telescope.nvim/lua/telescope/actions/mt.lua:65: in function 'key_func'
	...hare/nvim/lazy/telescope.nvim/lua/telescope/mappings.lua:269: in function <...hare/nvim/lazy/telescope.nvim/lua/telescope/mappings.lua:268>