fix: consider the original config for signs

[delphinus]

It always overwrites signs' config this code below, and therefore, all user settings are ignored.

ale/lua/ale/diagnostics.lua

Line 68 in 65b49c1

signs = {priority = vim.g.ale_sign_priority }

With this patch, ALE considers user settings and overwrites priority only if needed.

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The feature for retrieving the current config values for diagnostics has been implemented in neovim/neovim#17045. This patch is included in Neovim v0.7.0 and higher, so this fix cannot work with v0.6.x -- the minimum version ALE supports. For v0.6.x, I did a small fix (1485bab).

[delphinus]

@w0rp The current minimum version to support is Neovim 0.6.0. I found this was determined in #4345 because 0.6.0 is the first version that implemented vim.diagnostic.

Can you bump up this to 0.7.0 or higher? I want to fix this problem in this PR, but it needs the feature in 0.7.0 (details are written in PR's description).

[hsanson]

@delphinus will try to tackle this during holidays: #4887