Improve parse error for is <string-lit>

[john-h-kastner-aws]

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Bug Category

Cedar Parser

Describe the bug

Parsing the policy permit(principal, action, resource) when { principal is "User" }; gives the error

[jkastner@dev-dsk-jkastner-1a-3309db3b cedar]$ cargo run -q check-parse <<<'permit(principal, action, resource) 
when { principal is "User" };'
  × failed to parse policy set
  ╰─▶ right hand side of an `is` expression must be an entity type name, but got `User`
   ╭────
 1 │ permit(principal, action, resource) when { principal is "User" };
   ·                                                         ──────
   ╰────
  help: try using `==` to test for equality

This error should be more useful:

1. In the message text User should be rendered as a string "User"
2. The try using `==` to test equality help text is not so useful because in this case it's just a likely that they meant to drop the quotes for principal is User.

Expected behavior

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Reproduction steps

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Code Snippet

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Log output

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Additional configuration

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Operating System

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Additional information and screenshots

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