Fix identifier parsing of human-readable schemas

[shaobo-he-aws]

Description of changes

• Fixes Human schema parser incorrectly rejects schemas #913
• Fixes parsing errors of schemas that use in as identifiers: "unexpected token in" to "in is a reserved identifier"

Issue #, if available

#913

Checklist for requesting a review

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

• A bug fix or other functionality change requiring a patch to cedar-policy.

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 formal model or DRT infrastructure.