CSS isolation: Razor compiler update to emit CSS scope attributes

[SteveSandersonMS]

This is one of the work items needed for #10170

Given an extra optional "scope ID" string for each .razor file being compiled (which is to be supplied by other work external to this issue), we need the Razor compiler to generate an extra call like:

builder.AddAttribute(seq, ScopeId, null); // adds an empty attribute named ScopeId

... for every HTML element being rendered by that component. This applies equally across regular elements, elements inside RenderFragments, and elements that get optimized into pre-stringified HTML markup fragments.