fix(types): export LanguageType utility type

[metonym]

Fixes #373

LanguageType is a utility for instrumenting custom language implementations for highlight.js. It's meant to be used by consumers, so this should be exported.

interface LanguageType<TName extends string> {
  name: TName;
  register: LanguageFn;
}

Example usage:

import type { LanguageType } from "svelte-highlight";

const language: LanguageType<"custom-language"> = {
  name: "custom-language",
  register: () => {
    return {
      contains: [],
      tokenize: () => {
        return {
          tokens: [],
        };
      },
    };
  },
};