Range and StartRange methods on Expression

This gives us some contextual information to use for error handling when
working generically with expressions.

zcl/json/structure.go

@@ -259,3 +259,11 @@ func (e *expression) Value(ctx *zcl.EvalContext) (cty.Value, zcl.Diagnostics) {
 		return cty.DynamicVal, nil
 	}
 }
+
+func (e *expression) Range() zcl.Range {
+	return e.src.Range()
+}
+
+func (e *expression) StartRange() zcl.Range {
+	return e.src.StartRange()
+}

zcl/structure.go

@@ -101,6 +101,9 @@ type Expression interface {
 	// the specific symbol in question.
 	Value(ctx *EvalContext) (cty.Value, Diagnostics)
 
+	Range() Range
+	StartRange() Range
+
 	// TODO: A "Variables" method that returns a description of all of the
 	// variables used in the expression, so callers can populate the scope
 	// only with variables that are actually used.