[json] don't suggest top level snippet if there's already an object

Issue Type: <b>Bug</b> - Open one of the user snippets JSON files - Type " to bring up the empty snippet suggestion - Tab to insert it The provided snippet inserts top-level curly braces, which (I don't know JSON) an existing snippet file already has, making the new snippet invalid. ```JSON // Valid { "Existing Snippet": { "prefix": "my_snippet", "body": "this is a snippet", "description": "I'm an existing snippet" } } // "End of file expected" warning on the next line { "snippetName": { "prefix": "prefix", "body": "snippet", "description": "description" } } ``` VS Code version: Code 1.24.1 (24f62626b222e9a8313213fb64b10d741a326288, 2018-06-13T17:51:32.889Z) OS version: Windows_NT x64 6.3.9600 <details> <summary>System Info</summary> |Item|Value| |---|---| |CPUs|Intel(R) Core(TM) i7-3630QM CPU @ 2.40GHz (8 x 2395)| |GPU Status|2d_canvas: enabled<br>flash_3d: enabled<br>flash_stage3d: enabled<br>flash_stage3d_baseline: enabled<br>gpu_compositing: enabled<br>multiple_raster_threads: enabled_on<br>native_gpu_memory_buffers: disabled_software<br>rasterization: disabled_software<br>video_decode: enabled<br>video_encode: enabled<br>vpx_decode: unavailable_software<br>webgl: enabled<br>webgl2: enabled| |Memory (System)|15.95GB (10.49GB free)| |Process Argv|C:\Program Files\Microsoft VS Code\Code.exe| |Screen Reader|no| |VM|0%| </details><details><summary>Extensions (2)</summary> Extension|Author (truncated)|Version ---|---|--- vscode-lua|gcc|0.1.2 color-highlight|nau|2.3.0 (1 theme extensions excluded) </details> <!-- generated by issue reporter -->