Improve parse errors around invalid is expressions

[john-h-kastner-aws]

Description of changes

Fix #409 by parsing an Add rather than an Name in is expression. This allows for parsing expression in this position so that principal is User::"alice" can parse, allowing for a more useful error in when building the ast. The Add production needs to be used instead of Expr to avoid ambiguity with the .. is .. in .. form.

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[shaobo-he-aws]

Nit: maybe we should add a comment here saying Add shows up here rather than Name because we want better error message. Otherwise it could be confusing here because the grammar specifies that a Name comes after is.