JS Doc type intellisense not working when using custom file extension

From @csvenke on September 30, 2016 20:52

• VSCode Version: 1.5.3
• OS Version: Windows 10

In a .js file, intellisense provides correct jsdoc types

In a custom extension file associated with javascript, types from jsdoc are not provided

Steps to Reproduce:

1. Create a .sj file
2. In settings.json, set "files.associations": { "*.sj": "javascript" }
3. Create a function with jsdoc parameters with types
4. Call function and check if intellisense provides types

Copied from original issue: Microsoft/vscode#13100

使用 .sj 文件、settings.json 中的 files.associations 设置以及带有 JSDoc 类型的函数重现该问题。首先跟踪 JavaScript language service 如何处理关联的自定义扩展名；完成标准是，.sj 文件中的 JSDoc 参数类型能够像 .js 文件中一样显示在 IntelliSense 中。