refactor: more accurate interface design and terminology

[johnsoncodehk]

Try to reduce the abstraction of core parts as much as possible, and use more precise terminology.

language-core uses the term "Script" instead of "File", because "File" implies dependence on the File System and "Script" semantics are looser.

language-core

• Renamed SourceFile interface to SourceScript
• LanguageContext interface and FileRegistry interface are merged into Language interface
• Removed FileRegistry.getVirtualCode API, now you should use:

   Language.scripts.get(scriptId).generated.embeddedCodes.get(virtualCodeId)

language-service

• Renamed ServicePlugin interface to LanguageServicePlugin
• Renamed Result interface to ProviderResult
• Renamed LanguagePlugin.typescript's getScript hook to getServiceScript
• Renamed LanguagePlugin.typescript's getExtraScripts hook to getExtraServiceScripts
• Removed ServiceContext.documents.getVirtualCodeByUri, now you should use:

   const decoded = context.decodeEmbeddedDocumentUri(documentUri);
   const sourceScript = decoded && context.language.scripts.get(decoded[0]);
   const virtualCode = decoded && sourceScript?.generated?.embeddedCodes.get(decoded[1]);

• Removed ServiceContext.documents.getVirtualCodeUri, now you should use:

   context.encodeEmbeddedDocumentUri(documentUri, virtualCodeId);