Only merge top-level array when merging languages.toml

[the-mikedavis]

history: this is a revert of the revert of #2145 with a fix on top. #2145 accidentally took out all array merging.

I got a bit confused when sorting out a merge conflict when rebasing #2145: I thought the call in helix-loader/src/config.rs:22 was merging config.toml and not languages.toml 😅

So that boolean was just wrong and that was preventing the top-level array in languages.toml with user-specified languages.toml overrides. I gave this a spin locally with

[[language]]
name = "erlang"
auto-format = true

and all appears to be well 👍

closes #1000

[the-mikedavis]

This also does not hang infinitely for me with the long languages.toml #2187 (comment)