Give raw .hs files a language server

[thomasjm]

extensions = if raw then ["hs"] else [] meant that with the default raw = false the HLS config claimed no file extensions, so a plain .hs file in a sandbox reached no language server while notebooks worked. rust-analyzer keeps extensions = ["rs" "rlib"] in both modes and varies only notebook_suffix; HLS now does the same.

Confirmed against the generated config in a built environment: extensions goes from [] to ["hs"], with notebook_suffix still "" in non-raw mode.

The test doesn't cover that change — the suite launches a named language server directly, so it passes either way. What it does cover is that a bare .hs survives hnls, which passes non-notebook uris through untouched, and gets real hover from HLS.

Closes part 3 of #120.