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

[StephenWeatherford]

Issue Type: Bug

See microsoft/vscode-azurearmtools#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 #69868 (comment)). Unfortunately, that means vscode doesn't know we're associated with json files.

VS Code version: Code - Insiders 1.48.0-insider (e7920dc, 2020-07-10T11:55:11.286Z)
OS version: Darwin x64 19.5.0

System Info

Item	Value
CPUs	Intel(R) Core(TM) i7-9750H CPU @ 2.60GHz (12 x 2600)
GPU Status	2d_canvas: enabled flash_3d: enabled flash_stage3d: enabled flash_stage3d_baseline: enabled gpu_compositing: enabled metal: disabled_off multiple_raster_threads: enabled_on oop_rasterization: disabled_off opengl: enabled_on protected_video_decode: unavailable_off rasterization: enabled skia_renderer: disabled_off_ok video_decode: enabled viz_display_compositor: enabled_on webgl: enabled webgl2: enabled
Load (avg)	3, 3, 3
Memory (System)	16.00GB (0.80GB free)
Process Argv
Screen Reader	no
VM	0%

Extensions (5)

Extension	Author (truncated)	Version
gitlens	eam	10.2.2
todo-tree	Gru	0.0.178
vscode-dotnet-runtime	ms-	0.1.2
vscode-typescript-tslint-plugin	ms-	1.2.3
vscode-todo-highlight	way	1.0.4

[alexdima]

@aeschli @bpasero Perhaps we should introduce a new property in package.json for languages, called "defaultFileName" or "defaultFileExtension" or something like that and that could be picked up by the Save dialog.

Today the Save dialog relies on the "extensions" property, but that one was introduced for the mapping in the other direction, to determine a language id from a file name and not the other way around.

Related: #102032