Evaluates a parsed AST against a context (map of field names to values).

Example:

context = %{
  "x" => Decimal.new("100.00"),
  "y" => Decimal.new("20.00"),
  "category" => "B",
  "z" => Decimal.new("5.00")
}

{:ok, ast} = Dsqlex.Parser.parse(tokens)
{:ok, result} = Dsqlex.Evaluator.evaluate(ast, context)