feat: user can custom the format of diagnostic rendering

[adoyle-h]

No description provided.

[adoyle-h]

@folke Could you review this PR? It has been waiting for one month.

[folke]

Thank you for the suggestion, but I'm not a fan of this. You can always just override the method yourself.

local renderer = require("trouble.renderer")

function renderer.render_diagnostics(view, text, items)
  -- add your custom code here
end

[adoyle-h]

@folke But the signs variable is unexported.
How to local sign = diag.sign or signs[string.lower(diag.type)] in my override function renderer.render_diagnostics?

[folke]

I just pushed a change that exposes renderer.signs