[json] completion has bad replacement span, overwrites comment

Issue Type: <b>Bug</b> * Open a `tsconfig.json` containing: ```json { "compilerOptions": { "target": "es5" } } ``` * Comment out `//"es5"` * Put a `"` in front of it. Choose the completion that pops up for "es2015". * Result: ```json { "compilerOptions": { "target": "es2015",es5" } } ``` VS Code version: Code - Insiders 1.22.0-insider (6b4d53cdab8bcae1eaaa4934d93c077319b573db, 2018-04-04T13:52:38.747Z) OS version: Windows_NT x64 10.0.15063 <details> <summary>System Info</summary> |Item|Value| |---|---| |CPUs|Intel(R) Xeon(R) CPU E5-1620 v3 @ 3.50GHz (8 x 3492)| |Memory (System)|15.91GB (6.76GB free)| |Process Argv|C:\Program Files\Microsoft VS Code Insiders\Code - Insiders.exe| |Screen Reader|no| |VM|0%| </details>Extensions: none <!-- generated by issue reporter -->