Introduce preprocess config prop

[jarda-svoboda]

This config prop should define a preprocess strategy (or a custom preprocess function). Preprocessor runs immediately after the translation data load.

Let's say we have a translation input like this:

{"a": {"b": [{"c": {"d": 1}}, {"c": {"d": 2}}]}}

'full' (default) setting will result in:

{"a.b.0.c.d": 1, "a.b.1.c.d": 2}

'preserveArrays' in:

{"a.b": [{"c.d": 1}, {"c.d": 2}]}

'none' (nothing's changed):

{"a": {"b": [{"c": {"d": 1}}, {"c": {"d": 2}}]}}

• Introduce preprocess config prop
  Closes Can Arrays be translated? lib#92
• Update tests
• Update docs
• Add JSDoc comments to declarations
• Add an example (will be added later under separate PR on sveltekit-i18n/lib)
• Update dependencies
  Fixes Not ready for Svelte 4.0 lib#113