Clean up findPrecedingToken and avoid returning whitespace-only jsx text token (#21903)

src/compiler/utilities.ts

@@ -5153,6 +5153,7 @@ namespace ts {
     /**
      * True if node is of some token syntax kind.
      * For example, this is true for an IfKeyword but not for an IfStatement.
+     * Literals are considered tokens, except TemplateLiteral, but does include TemplateHead/Middle/Tail.
      */
     export function isToken(n: Node): boolean {
         return n.kind >= SyntaxKind.FirstToken && n.kind <= SyntaxKind.LastToken;