Shell: Add support for brace expansions

This adds support for (basic) brace expansions with the following
syntaxes:
- `{expr?,expr?,expr?,...}` which is directly equivalent to `(expr expr
  expr ...)`, with the missing expressions replaced with an empty string
  literal.
- `{expr..expr}` which is a new range expansion, with two modes:
    - if both expressions are one unicode code point long, the range is
      equivalent to the two code points and all code points between the
      two (numerically).
    - if both expressions are numeric, the range is equivalent to both
      numbers, and all numbers between the two.
    - otherwise, it is equivalent to `(expr expr)`.

Closes #3832.

Shell/NodeVisitor.cpp

@@ -55,6 +55,12 @@ void NodeVisitor::visit(const AST::BarewordLiteral*)
 {
 }
 
+void NodeVisitor::visit(const AST::BraceExpansion* node)
+{
+    for (auto& entry : node->entries())
+        entry.visit(*this);
+}
+
 void NodeVisitor::visit(const AST::CastToCommand* node)
 {
     node->inner()->visit(*this);
@@ -153,6 +159,12 @@ void NodeVisitor::visit(const AST::Pipe* node)
     node->right()->visit(*this);
 }
 
+void NodeVisitor::visit(const AST::Range* node)
+{
+    node->start()->visit(*this);
+    node->end()->visit(*this);
+}
+
 void NodeVisitor::visit(const AST::ReadRedirection* node)
 {
     visit(static_cast<const AST::PathRedirectionNode*>(node));