PublicShow -- Highlight token server

This module provides the Prolog part of server-assisted highlighting for SWISH. It is implemented by managing a shadow copy of the client editor on the server. On request, the server computes a list of semantic tokens.

To be done
- Use websockets
Source current_highlight_state(?UUID, -State) is nondet
Return info on the current highlighter
Source prolog:xref_open_source(+UUID, -Stream)[multifile]
Open a source. As we cannot open the same source twice we must lock it. As of 7.3.32 this can be done through the prolog:xref_close_source/2 hook. In older versions we get no callback on the close, so we must leave the editor unlocked.
Source swish_config:config(-Name, -Styles) is nondet[multifile]
Provides the object, a JSON object that maps style properties of user-defined extensions of library(prolog_colour). This info is used by the server-side colour engine to populate the CodeMirror styles.
To be done
- Provide summary information
 css(?Context, ?Selector, -Style) is nondet[multifile]
Multifile hook to define additional style to apply in a specific context. Currently defined contexts are:
Used for CodeMirror hover extension.
Selector- is a CSS selector, which is refined by Context
Style- is a list of Name(Value) terms.
Source token_info(+Token:dict)// is det[multifile]
Generate HTML, providing details about Token. Token is a dict, providing the enriched token as defined by style/3. This multifile non-terminal can be hooked to provide details for user defined style extensions.