fix(localize): allow ICU expansion case to start with any character except } (#36123)

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

packages/compiler/src/ml_parser/lexer.ts

@@ -747,7 +747,7 @@ function isNamedEntityEnd(code: number): boolean {
 }
 
 function isExpansionCaseStart(peek: number): boolean {
-  return peek === chars.$EQ || chars.isAsciiLetter(peek) || chars.isDigit(peek);
+  return peek !== chars.$RBRACE;
 }
 
 function compareCharCodeCaseInsensitive(code1: number, code2: number): boolean {