Validate placeholders in locales against default locale

[xPaw]

For example:

english:

	"app_lowest_price": {
		"message": "SteamDB lowest recorded price is <b>$price$</b>",
		"placeholders": {
			"price": {
				"content": "$1",
				"example": "1,99€"
			}
		}
	},

another locale:

	"app_lowest_price": {
		"message": "Минимальная зафиксированная цена на SteamDB — <b>$price$</b>",
		"placeholders": {
			"price": {
				"content": "$2",
				"example": "1,99€"
			}
		}
	},

I would like this produce a warning because the placeholder in the non default locale uses "$2" instead of "$1".

[rpl]

We briefly discussed about this issue in our triage meeting today and agreed that this would be a reasonable enhancement to the locale files validation.

The idea could be to validate that the placeholders that are used in the en-US locale are being matched by the placeholders included in the other locales.

As a side note, introducing that kind of additional check may likely require changes to JSONScanner, which is defined in https://github.com/mozilla/addons-linter/blob/c3fa9a64507957821f58138c1fdd2d186c2987a8/src/scanners/json.js