Such errors are raised when SyntaxError nodes are executed, and are also
used for internal control flow.
The 'break' and 'continue' commands are currently only allowed inside
for loops, and outside function bodies.
This also adds a 'loop' keyword for infinite loops.
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.
