Support new ARM Language server

Hi GoEddie,

I am a PM on the ARM template deployments team at Microsoft.

In September, the Azure Resource Manager Tools extension (azurerm-vscode-tools, repo at https://github.com/microsoft/vscode-azurearmtools) will be updated to introduce a new language server for "ARM" deployment template files. When the extension is installed, deployment templates will automatically be detected via the $schema and the editor language will be set to "arm-deployment" to enable this new language server.

**How this affects other extensions that work with deployment templates files**
Any extension that wants to have an editing experience for ARM templates should support the "arm-deployment" language ID instead of (if it depends on azurerm-vscode-tools being installed), or in addition to json/jsonc (if it wants to run with or without the azurerm-vscode-tools extension being installed).

Let us know if you have any questions,
Alex