Add recursive substitution of config variables

[timiimit]

I am using CMake Tools extension which allows to set cmake.buildDirectory config variable.
I wanted to specify location of compile_commands.json through settings like so:

"clangd.arguments": [
	"--compile-commands-dir=${config:cmake.buildDirectory}"
],

but I noticed that variables get resolved only once. cmake.buildDirectory can in turn contain another variable such as ${workplaceFolder} in my case. For this, a recursive resolution would be needed.

I'm not sure whether a recursion limit would be desired or if PR breaks in any weird cases.