feat: extmarks copy filtering support

[nowaylifer]

Callback to filter out extmarks that should not be copied over. Adding this because I needed a way to hide indent-blankline in context window.

{
    "nvim-treesitter/nvim-treesitter-context",
    opts = {
        filter_extmarks = function(extmark)
          return extmark[4].ns_id ~= vim.api.nvim_get_namespaces()["indent_blankline"]
        end
      }
  },