Previously, an expansion case could only start with an alpha numeric character.
This commit fixes this by allowing an expansion case to start with any character
except `}`.
The [ICU spec](http://userguide.icu-project.org/formatparse/messages) is pretty vague:
> Use a "select" argument to select sub-messages via a fixed set of keywords.
It does not specify what can be a "keyword" but from looking at the surrounding syntax it
appears that it can indeed be any string that does not contain a `}` character.
Closes#31586
PR Close#36123
fced8ee40e