refacto: expose a function to set custom capture for the higlighter

BREAKING: deprecate custom_captures from highlight config.
This allows plugin authors to extend the map. It also avoids settings
user configuration specific values during the highlighter
initialization (SOC).
Not sure how much value this brings, and might potentially break a lot
of people configurations. This is questionable.

lua/nvim-treesitter/configs.lua

@@ -22,8 +22,9 @@ local is_initialized = false
 local builtin_modules = {
   highlight = {
     module_path = "nvim-treesitter.highlight",
-    enable = false,
+    -- @deprecated: use `highlight.set_custom_captures` instead
     custom_captures = {},
+    enable = false,
     is_supported = function(lang)
       return queries.has_highlights(lang)
     end,