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/Forward.h

@@ -41,6 +41,7 @@ class And;
 class ListConcatenate;
 class Background;
 class BarewordLiteral;
+class BraceExpansion;
 class CastToCommand;
 class CastToList;
 class CloseFdRedirection;
@@ -58,6 +59,7 @@ class Join;
 class MatchExpr;
 class Or;
 class Pipe;
+class Range;
 class ReadRedirection;
 class ReadWriteRedirection;
 class Sequence;