Enable prettier to work against adx-template-json files

[smithgp]

With the fix for #5, to workaround microsoft/vscode#73424 not being implemented, the prettier vscode formatter will no longer format template json files, since those are now 'adx-template-json' languageId instead of 'json' or 'jsonc'.

We need to make it so prettier, when used in the vscode project, can be configured to format template json files, using prettier's default json formatting plugin and config.

Currently, I see how we could do it by directly modifying the vscodeLanaugeIds in prettier (e.g. in https://github.com/prettier/prettier/blob/master/src/language-js/index.js). Also, prettier/prettier-vscode#597 sound like what we want, but got closed pretty quickly w/o a fix.

I would like to have a vscode-only fix, either in our extension or in prettier/prettier-vscode, since the adx-template-json language id is currently only going to live in our vscode extension, and using prettier cli directly against template json files should work just fine.