#48in24: Add analyzer feedback for Parallel Letter Frequency

The Parallel Letter Frequency exercise on the Java track [is being featured in the #48in24 challenge](https://exercism.org/challenges/48in24/implementation_status), which means that we expect an influx of students attempting to solve it during that week.

It would be nice if the exercise contains some more content by that time.
One example of this is to add [analyzer](https://exercism.org/docs/building/tooling/analyzers) feedback for the exercise, to provide automated feedback on submitted solutions.

Adding analyzer feedback is done in three steps:

1. Create (or update) the `.meta/design.md` file for the exercise explaining what type of feedback the analyzer should provide ([example for the Leap exercise](https://github.com/exercism/java/blob/main/exercises/practice/leap/.meta/design.md)).
2. Add the copy for the analyzer comments to [exercism/website-copy](https://github.com/exercism/website-copy/tree/main/analyzer-comments/java).
3. Implement the analyzer rules in [exercism/java-analyzer](https://github.com/exercism/java-analyzer).

首先定位 Parallel Letter Frequency 练习并查看其 `.meta/design.md`，以 Leap 练习设计作为参考示例。然后检查 `analyzer-comments/java` 下的 `exercism/website-copy` 以及 `exercism/java-analyzer` 仓库。完成的标准是：按照所描述的三个步骤，实现反馈设计、analyzer 注释文本和相应的 analyzer 规则。

由索引模型根据 Issue 内容生成。