Recipe: register custom highlighting

Register a language or theme before the first surface uses it:

import { registerCustomLanguage, registerCustomTheme } from '@pierre/diffs';

registerCustomLanguage(
  'my-language',
  () => import('./my-language.tmLanguage.json'),
  ['myext']
);

registerCustomTheme('my-theme', () => import('./my-theme.json'));

Set file.lang to the custom language name. Set options.theme to the custom theme name. Use registerCustomCSSVariableTheme when CSS variables supply the theme colors.