[json] allow $ref in confiuguration schemas

Issue Type: <b>Bug</b> Steps to Reproduce: 1. Create a ["definitions"](https://json-schema.org/understanding-json-schema/structuring.html) member for a JSON schema, and put a subschema there. 2. Attempt to refer to this subschema in `settings.json`. *** I'm in a folder called "json" with a subfolder called "schemas" with the schemas, and a subfolder called ".vscode" for the `settings.json`. Here is the definitions schema I am using: ``` { "$schema": "https://json-schema.org/draft/2019-09/schema", "type": "object", "definitions": { "items": { "type": "array", "items": { "$ref": "items.json#" }, "minItems": 1 } } } ``` If I just make `items.json` into an array and use `"url": "./schemas/items.json"` works fine, but that isn't very conducive to my current work case. &nbsp; So I tried using this: ``` "schema": { "type": "array", "items": { "$ref": "./schemas/items.json" } } ``` Got this error: "Problems loading reference 'vscode://schemas/custom/schemas/items.json': Request vscode/content failed with message: cannot open vscode://schemas/custom/schemas/items.json. Detail: resource is not available". &nbsp; Tried using this: `"url": "file:///C:/Users/BLATANT-DOXX-RISK/Source/Repos/json/schemas/types.json#/definitions/items"` And it just used `types.json`, ignored the definitions part, and looked for nothing but a blank object. &nbsp; Finally, I tried using this: `"url": "./schemas/types.json#/definitions/items"`. Got this error: "File not found (c:\Users\BLATANT-DOXX-RISK\Source\Repos\json\schemas\types.json#\definitions\items))". &nbsp; @aeschli came up a *lot* when looking through Github issues. *** VS Code version: Code 1.38.1 (b37e54c98e1a74ba89e03073e5a3761284e3ffb0, 2019-09-11T13:35:15.005Z) OS version: Windows_NT x64 10.0.18362 <details> <summary>System Info</summary> |Item|Value| |---|---| |CPUs|Intel(R) Core(TM) i7-6700HQ CPU @ 2.60GHz (8 x 2592)| |GPU Status|2d_canvas: enabled<br>flash_3d: enabled<br>flas…