feat: add language-filter option

[JRasmusBm]

Why is the change needed?

I define similar configuration for a bunch of similar filetypes. This leads to duplicates in the configuration list.

How is the need addressed?

Support a language_filter option that can be used as follows:

  require("telescope").extensions.dap.configurations {
    language_filter = function(lang)
      return lang == vim.bo.filetype
    end,
  }

Side-effects of the changes:

• Change variable naming to be consistent with the usage.

[Conni2461]

thanks :)