wdl-lint: add NoHostPathLiterals rule

[raman976]

Fixes #718

This PR adds the NoHostPathLiterals lint rule to flag File and Directory declarations in input and private sections that use absolute host path literals as defaults. Absolute paths resolve on the host filesystem, making tasks non-portable across environments.

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[raman976]

Hi @claymcleod , please review this PR and let me know if there are any changes that need to be done. I would be happy to do them.