TS Server plugin extension contributions

[mjbvz]

Background
TS Server plugins allow developers to extend TS Server's editor features. This plugin for example adds syntax checking and intellisense for styled-components. Using these plugins in VS Code currently requires a number of manual setup steps, including npm installing the plugin and adding it to your jsconfig

We would like to eliminate these steps by allowing VS Code extensions to contribute TS Server plugins, just like they can contribute new grammars or commands. A contributed plugin would always be loaded when you use VS Code's version of TypeScript

Proposal
In an extension's package.json, add a new typescriptServerPlugins contribution point:

 "contributes": {
    "typescriptServerPlugins": [
      {
        "name": "typescript-styled-plugin"
      }
    ]
  }

typescriptServerPlugins is array of plugins to load. The name field specifies the name of the plugin to load from within the extension (for the above example, the extension would have a dependency on typescript-styled-plugin which would be installed in its node_modules)

The typescript-styled-plugin plugin would now be loaded automatically for JS and TS (both for code with and without a jsconfig or tsconfig). It would not be loaded if you are using a custom typescript version due to concerns about versioning (this area needs more thought).