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 {

packages/compiler/test/ml_parser/html_parser_spec.ts

@@ -313,6 +313,26 @@ import {humanizeDom, humanizeDomSourceSpans, humanizeLineColumn} from './ast_spe
           expect(p.errors.length).toEqual(0);
         });
 
+        it(`should support ICU expressions with cases that contain any character except '}'`,
+           () => {
+             const p = parser.parse(
+                 `{a, select, b {foo} % bar {% bar}}`, 'TestComp', {tokenizeExpansionForms: true});
+             expect(p.errors.length).toEqual(0);
+           });
+
+        it('should error when expansion case is not properly closed', () => {
+          const p = parser.parse(
+              `{a, select, b {foo} % { bar {% bar}}`, 'TestComp', {tokenizeExpansionForms: true});
+          expect(humanizeErrors(p.errors)).toEqual([
+            [
+              6,
+              'Unexpected character "EOF" (Do you have an unescaped "{" in your template? Use "{{ \'{\' }}") to escape it.)',
+              '0:36'
+            ],
+            [null, 'Invalid ICU message. Missing \'}\'.', '0:22']
+          ]);
+        });
+
         it('should error when expansion case is not closed', () => {
           const p = parser.parse(
               `{messages.length, plural, =0 {one`, 'TestComp', {tokenizeExpansionForms: true});