Prior to this commit, the SpEL Tokenizer threw an IllegalStateException
when an unsupported character was detected in a SpEL expression;
however, the Javadoc for ExpressionParser.parseExpression() states that
it throws a ParseException if an exception occurred during parsing.
This commit addresses that issue by throwing a SpelParseException for
an unsupported character in a SpEL expression, using a new
UNSUPPORTED_CHARACTER enum constant in SpelMessage.
Closes gh-33767
c98f314665