ifupdown_ng.config: Refactor parser state-tracking to a new module

This will allow other files to benefit from the same parser state model.

Signed-off-by: Kyle Moffett <kyle@moffetthome.net>
ifupdown_ng.config: Shard into a new sub-package

The first step is just splitting the tokenizer into its own package, but
eventually we will split all the other pieces of the config into their
own files too.

This is necessary to make it possible for modules to define new config
keywords, blocks, subblocks, etc.

Signed-off-by: Kyle Moffett <kyle@moffetthome.net>