Improve the type of TemplateExpression

[bakkot]

TemplateExpression has

interface TemplateExpression : Expression {
  attribute Expression? tag;
  attribute (Expression or TemplateElement)[] elements;
}

but that's imprecise: the elements must alternate, be nonempty, and begin and end with TemplateElement. A better type would be

interface TemplatePart {
  attribute Expression expression;
  attribute TemplateElement string;
}
interface TemplateExpression : Expression {
  attribute Expression? tag;
  attribute TemplateElement first;
  attribute TemplatePart[] parts;
}

which represents precisely the legal templates.