No way for an extension to tell have vscode suggest .JSON when user saves file if it is not the main extension for json files

Issue Type: <b>Bug</b> See https://github.com/microsoft/vscode-azurearmtools/issues/809 (Save dialog for arm-template files should suggest saving with JSON extension like json files do) 1. Install https://marketplace.visualstudio.com/items?itemName=msazurermtools.azurerm-vscode-tools 2. Create a new file (don't save) 3. Change the language to "ARM Resource Manager Template" 4. CTRL+S !) the dialog doesn't give a suggestion for extension EXPECTED: vscode should suggest .JSON for the extension, but there appears to be no way to get that to happen. MORE INFO: Our package.json info looks like this for the arm-template language id: ``` "languages": [ { "id": "arm-template", "aliases": [ "Azure Resource Manager Template" ], "configuration": "dist/grammars/jsonc.arm.language-configuration.json" } ], ``` I can't put .json as our extension, because if I do, then *all* json files opened will use the arm-template language. Rather, we manually switch to arm-template when we detect a specific schema in a json file (`https://schema.management.azure.com/schemas/2019-04-01/deploymentTemplate.json#`). This is the mechanism that we were told to use because vscode doesn't provide a way to detect language ID based on any content past the first line (see https://github.com/microsoft/vscode/issues/69868#issuecomment-490084449). Unfortunately, that means vscode doesn't know we're associated with json files. VS Code version: Code - Insiders 1.48.0-insider (e7920dce7bfd0b707ebfc0a5379c6edd2233e475, 2020-07-10T11:55:11.286Z) OS version: Darwin x64 19.5.0 <details> <summary>System Info</summary> |Item|Value| |---|---| |CPUs|Intel(R) Core(TM) i7-9750H CPU @ 2.60GHz (12 x 2600)| |GPU Status|2d_canvas: enabled<br>flash_3d: enabled<br>flash_stage3d: enabled<br>flash_stage3d_baseline: enabled<br>gpu_compositing: enabled<br>metal: disabled_off<br>multiple_raster_threads: en…