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.

README.md

@@ -273,10 +273,6 @@ Consistent syntax highlighting.
 require'nvim-treesitter.configs'.setup {
   highlight = {
     enable = true,
-    custom_captures = {
-      -- Highlight the @foo.bar capture group with the "Identifier" highlight group.
-      ["foo.bar"] = "Identifier",
-    },
     -- Setting this to true will run `:h syntax` and tree-sitter at the same time.
     -- Set this to `true` if you depend on 'syntax' being enabled (like for indentation).
     -- Using this option may slow down your editor, and you may see some duplicate highlights.
@@ -286,6 +282,17 @@ require'nvim-treesitter.configs'.setup {
 }
 ```
 
+You can add custom highlight captures with:
+
+```vim
+lua <<EOF
+  require"nvim-treesitter.highlight".set_custom_captures {
+    -- Highlight the @foo.bar capture group with the "Identifier" highlight group.
+    ["foo.bar"] = "Identifier",
+  }
+EOF
+```
+
 #### Incremental selection
 
 Incremental selection based on the named nodes from the grammar.