`is` expression does not parse when the entity type is a variable name

[john-h-kastner-aws]

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Bug Category

Cedar Parser

Describe the bug

In the following expressions, the entity uid parses correctly but we fail to parse the is expression because we treat the right side as an variable expression instead of an entity type name. These expressions should parse without error.

principal::"foo" is principal
action::"foo" is action
resource::"foo" is resource
context::"foo" is context