Improve custom configuration variables

@sean-mcmanus I think my comment is already a suggestion to make it better:

1. Allow customConfigurationVariables to have property values which are arrays of strings;
2. Add a new class of VS Code variables which can be used in tasks.json and launch.json. These variables can be used in substitutions with a syntax like ${cpptools:<Variablename>}. This is similar to the syntax for environment variables, configuration variables, and command variables. These substitutions should also be allowed as part of an array of strings similar to the example for env variablles in c_cpp_properties.json files. So in arrays of strings the variable can not only represent a string but also an array of strings similar to the following lines in the mentioned example:
   "env": { "myDefaultIncludePath": ["${workspaceFolder}", "${workspaceFolder}/include"] },
"includePath": ["${myDefaultIncludePath}", "/another/path"]
   which is substituted to
   "includePath": ["${workspaceFolder}", "${workspaceFolder}/include", "/another/path"]

Originally posted by @PetaSoft in https://github.com/microsoft/vscode-cpptools/pull/5453#issuecomment-696612225

Inizia leggendo il riferimento collegato alle variabili di VS Code e l'esempio dello schema c_cpp_properties.json, concentrandoti su tasks.json, launch.json e customConfigurationVariables. Il lavoro è completato quando le variabili personalizzate con valori array funzionano e le sostituzioni di cpptools vengono espanse correttamente come stringhe o array nei campi di configurazione richiesti.