SSE: Add is_nan() and other like functions and allow '_' in func names (#43555)

* SSE: Add is_nan() and allow '_' in func names

is_null() infn(), is_inf(), is_number()

pkg/expr/mathexp/parse/lex.go

@@ -276,7 +276,7 @@ func lexSymbol(l *lexer) stateFn {
 func lexFunc(l *lexer) stateFn {
 	for {
 		switch r := l.next(); {
-		case unicode.IsLetter(r):
+		case unicode.IsLetter(r) || r == '_':
 			// absorb
 		default:
 			l.backup()