language js

`$fn($a)` where {
	$fn <: contains `where`,
	or {
		$a <: contains `'='`,
		$a <: contains `'=='`,
		$a <: contains `'!='`,
		$a <: contains `'!=='`
	},
	$a <: contains `null`,
	register_diagnostic(span=$a, message="Comparisons to null using '=' or '==' are not allowed. Use 'is' or 'is not' instead.", severity="error")
}