Issue Type: Feature Request

Currently, the typescript language service (tsserver) used by VS Code doesn't discover language service plugins installed locally. Instead, it tries to search them relative to a typescript sdk location.

Steps to reproduce:

1. Specify a globally installed typescript version to use for a workspace (in general, typescript must be installed outside of the local project).
2. Install a typescript language service plugin to local node modules (e. g. tslint-language-service).
3. Add language service plugin in tsconfig, e. g.
   "plugins": [ { "name": "tslint-language-service" } ]

Expected: Plugin is discovered and loaded.
Actual: Plugin is not discovered. TsServer tries to load it only relative to the typescript installation path.

Proposed change:
There is an option available in tsserver (--allowLocalPluginLoads) which enables discovery of locally installed plugins (available starting from TypeScript 2.3.3). Visual Studio already uses this option by default for TypeScript editing. The ideal solution would be to provide an option in VS Code to enable such behaviour (e. g. "typescript.tsserver.allowLocalPluginLoads": true) or enable it by default.

VS Code version: Code 1.21.0 (9a199d7, 2018-03-07T11:04:09.886Z)
OS version: Windows_NT ia32 10.0.16299