Add semantic token modifier for constant variables

[jmigual]

Feature Request

When using the clangd extension, it provides the readonly rule/token modifier (sorry I don't know the exact terminology) that can be used to highlight constant variables:

However, when using intellisense as the parser this does not happen as it only provides the local modifier:

(for colorblind people, the variable constVar has a dark blue color in the first image and has the same color as nonConst in the second one)

[sean-mcmanus]

This is being tracked on VS with https://developercommunity.visualstudio.com/t/Add-C-Local-Constants-to-Options--E/10335703