Some keywords need to control how the highlight query.
Use cases identified:

• morelike disable the highlight for perf reason
• in(source|title): may trigger regex support in the experimental highlighter or switch to source_text for insource:word
• subpageof: would like to indicate in the result page that a redirect has matched

This is not trivial for multiple reasons:

• currently what we display is chosen early in the process (ResultsType) but keywords may want to change that.
• not all features are available by all highlighters (experimental supports regex, others do not)
• there may be competing options: if two component target the redirect snippet, which one should we choose?

This may require some refactoring and additional subtasks depending on the complexity to achieve the initial goals (see use cases).