Add ConstFoldingRewriter to resolve const field references in expression method bodies

[Jeroen-VdB]

As mentioned in #250, expression method bodies that reference const fields were emitted as raw source text.

• Added ConstFoldingRewriter, a Roslyn syntax rewriter that substitutes const field references with their literal values before the expression is serialized to XML
• Hooked the rewriter into CompilerUtils.FindCode so it runs on every expression method body, before whitespace normalization and ToFullString()
  • Roslyn's semantic model already knows the value of every const field at compile time. The rewriter walks the syntax tree of the expression method, finds any MemberAccessExpression that resolves to a const field, and replaces it with the literal value (string, int, or bool). The existing path for const fields used directly as config parameters was already correct — this closes the gap for expression bodies.
• Added ConstFoldingRewriterTests with a string const folded across a separate class