Allow lines to start with `@foo` inside a block tag like `{@code}`

IIRC, we currently consider such an appearance of `@foo` to be an invalid tag (or, if it is a recognized one like `@return`, maybe a valid tag? or maybe never a valid tag because we're upset that the previous inline tag was never closed?). It is invalid... until Java 15, at which point https://bugs.openjdk.org/browse/JDK-8241780 saves us.

On those grounds, we don't format the Javadoc at all (which is not necessarily a great failure mode, but that's another issue).

Maybe we should just allow this, at least for cases that are not normally Javadoc tags, like `@Override`?

首先重現 issue 中描述的 Javadoc 區塊標籤案例，包括一行以 @Override 開頭的內容，並使用 JDK-8241780 作為 Java 15 行為的上下文。沒有指定檔案或測試；完成應表示 formatter 接受預期的 @foo 行，而不會將周圍的 inline 標籤視為無效。

由索引模型根據 Issue 內容生成。