'Fuzzy' file finding -- search on subsequence

(Note: leave for September branch)

This implements a 'fuzzy' finder for files through the `CTRL + .` file finder -- we now match on subsequences, rather than just the start of strings.

This allows, for example:

`rcle`

to match

`roclet-collate.R`

A scoring system is used to weigh matches as well, so the most likely intended results go on top.
IIUC, `boost::algorithm::contains` searches for matching substrings in a string, e.g. `ss` being a match in `missing`. I'm trying to do something a bit different, where e.g. `nv` would be a (case-insensitive) match in, for example, `NumericVector`. The idea being, the characters `nv` occur 'in that order' within `NumericVector`, so it's a potential match. I don't think boost handles this particular case (but I could be wrong?)