Highlight @example jsdoc body as source code

TS and JS Grammar Extension version: 0.0.47 / latest

Code
For a js doc body such as:

/**
 * @example
 * 1 + 1
 */
const a = 1

We mark the body of the @example block specially but do not actually syntax highlight it. I added an injection grammar in VS Code that does this: https://github.com/Microsoft/vscode/blob/master/extensions/typescript-basics/syntaxes/ExampleJsDoc.injection.tmLanguage.json

May be worth including in the core grammar. Let me know if you agree. The injection grammar will not work as is, we're relying on another injection grammar that handles separating the leading stars of the doc block from the documentation content itself

检查链接的 VS Code ExampleJsDoc.injection.tmLanguage.json，以及 repository 对 JSDoc 注释的 TypeScript/JavaScript grammar 处理。确认现有 injection 如何将开头的星号与文档内容分开；完成标准是 @example 块的正文能够获得源代码语法高亮。