Allow replacing fuzzy matching of completion/search results with prefix or substring matching.

[OlegOAndreev]

Right now FilterAndSortCompletionResponse uses CaseFoldingSubsequenceMatch for filtering out the completion candidates. This can add too many candidates and it would be nice to have an option to limit the completions to prefix or substring matches.

[jacobdufault]

Sure, PR welcome :)

[ddovod]

Hey guys. I would like to make some effort to add a couple more matching options. But ofc I need some help from you.

1. Is it okay to use external libraries (or just code chunks) with compatible licence?
   I would like to try these guys
   https://github.com/forrestthewoods/lib_fts/tree/master/code
   https://github.com/dsindex/autocomplete_libface
   https://github.com/xhochy/libfuzzymatch
   plus the most simple hand-written prefix matcher
2. I've found out that include completion also works not very good. So does include completion use FilterAndSortCompletionResponse?

[jacobdufault]

So long as the library is relatively light-weight and self contained I'll most likely be fine with adding it.

Yea, include completion uses FilterAndSortCompletionResponse.

[ddovod]

Hi. Here's pull request. Still wip, but please take a look #786