feat: add UnixFSPathSelectorBuilder

[rvagg]

Refactored and extracted out of Lassie and modified a bit for here.

Ref: filecoin-project/lassie#142

I've done two things aside from pulling code out of Lassie:

• Made the target selector something you have to provide, so you can do something custom at the target rather than the full bool we have in Lassie. But I've added some standard ones as globals here that can be easily used—we'll just use two of those for lassie.
• Added a matchPath option, like https://github.com/ipld/go-ipld-selector-text-lite/blob/1e69f951f8e30621f39f7090dac8c8f49200f1e4/parser.go#L40, to give the optionality to visit the path elements. I'm not sure if there's a great case for this but I don't really want to break the API by adding it later on and explore vs match is a feature of selectors as they are today.
• Rewired UnixFSPathSelector to use UnixFSPathSelectorBuilder underneath—this does have some subtle breaking changes, however: extraneous / are ignored—leading and trailing and duplicate, previously it did a strict split on / and treated the elements as necessary, even if they were "". If you look at the tests I added for this function, only leading slash optional passes on the old form because others have extraneous / characters that were strictly treated previously.

[rvagg]

This branch pulled back into lassie and used from there: filecoin-project/lassie#142 - note the use of the global SelectorSpecs from here in place of the original full bool.