Add highlight methods to search and search builder

[alexander-schranz]

This will add the core highlight methods for the highlighting search results:

$searchBuilder
    ->addIndex('blog')
    ->addFilter(new Condition\SearchCondition('Test'))
    ->highlight(
        fields: ['title', 'description'],
        preTag: '<mark>',
        postTag: '</mark>',
    )

fixes #343

See #349 for not yet rebased implementations.