fix: properly handle absolute paths in `CreatedModifiedDate` (#790)

When providing an absolute path to the content directory (e.g. when using an Obsidian Vault in another directory), the build step would fail with

    Failed to process `/absolute/path/to/file.md`: ENOENT: no such file or directory, stat '/current/working/directory/absolute/path/'

This problem originated in the `CreatedModifiedDate` transformer which tries to construct a native filesystem path to the file to call `fs.stat` on. It did not however, account for the original file path contained in the received `VFile` being an absolute path and so, just concatenated the current working directory with the absolute path producing a nonexistent one.

This patch adds a simple fix for this issue by checking if the original file path is already absolute before concatenating with the current working directory.