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

Start by reading the linked VS Code variables reference and the c_cpp_properties.json schema example, focusing on tasks.json, launch.json, and customConfigurationVariables. Done means array-valued custom variables work and cpptools substitutions expand correctly as strings or arrays in the requested configuration fields.