Behaviour for ~t sigil modifiers.

Implement this behaviour to extend the ~t sigil with per-call transformations: validate opts (init/1), compute the Gettext domain/context (domain_context/3), rewrite the message at compile time (preprocess/2), turn it into a plural form (pluralize/2), or transform the final translated string at runtime (postprocess/2).

All callbacks are optional. use GettextSigils.Modifier installs no-op defaults; override only what you need:

defmodule MyApp.UpcaseModifier do
  use GettextSigils.Modifier

  @impl true
  def postprocess(string, _opts), do: {:ok, String.upcase(string)}
end

Wire it up via the :modifiers option:

use GettextSigils,
  backend: MyApp.Gettext,
  sigils: [
    modifiers: [
      u: MyApp.UpcaseModifier,
      # with opts:
      s: {MyApp.ShoutModifier, intensity: 3}
    ]
  ]

Every callback returns {:ok, value} or {:error, reason}, where reason is either a string or any exception struct (e.g. a NimbleOptions.ValidationError). Exception structs are normalized to strings via Exception.message/1. init/1 errors raise a NimbleOptions.ValidationError at use GettextSigils time; all other errors raise ArgumentError at the respective compile or runtime stage.

See the Modifiers guide for full callback semantics, chaining rules, timing, and examples.