Fix var with keywords in namespace

[john-h-kastner-aws]

Description of changes

Follow on to #594. The parser also accepted variables like if::then::else::true::false::principal as long as all namespace identifiers were Cedar keywords and the last identifier was a valid variable.

Issue #, if available

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The change in this PR is (choose one, and delete the other options):

• A breaking change requiring a major version bump to cedar-policy (e.g., changes to the signature of an existing API).

I confirm that this PR (choose one, and delete the other options):

• Updates the "Unreleased" section of the CHANGELOG with a description of my change (required for major/minor version bumps).

I confirm that cedar-spec (choose one, and delete the other options):

• Does not require updates because my change does not impact the Cedar Dafny model or DRT infrastructure.

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