chore(nvim): add treesitter context plugin

Define a context line to show the context about the method/module one is
navigating.

config/nvim/lua/custom/plugins/init.lua

@@ -88,5 +88,6 @@ return {
         }
       })
     end
-  }
+  },
+  'nvim-treesitter/nvim-treesitter-context'
 }