Autocomplete: Prepare mixed retrieval examples

[philipp-spiess]

This PR adds two new context retrieval strategies that relies on mixing existing retrievers and changes how LSP light works:

1. bfg-mixed is a new setup that combines BFG retrieval and mixes it with the local jaccard similarity based scoring. This makes bfg more of an addition on top of local context rather then an alternative and should help for cases where BFG doesn't retrieve the best results yet. We can control it with an individual feature flag too.
2. local-mixed is a new setup that combines jaccard similiarty results with the section history retriever I added. I still think that there's something magical about just knowing which function you last changed and with this flag I want to find out if that's true

Additionally this changes lsp-light to mix with jaccard similiarty and not the section history only. We know that jaccard performs well and mixing with it should not make the result much worse (that would happen if we remove it)

Test plan

bfg mixed

local mixed

[olafurpg]

Exciting!