feat: add inconsistent_newlines rule

[adthrasher]

This pull request adds a new rule to wdl-lint.

• Rule Name: InconsistentNewlines

This rule checks for usage of both "\r\n" and "\n".

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• You have added the rule as an entry within RULES.md.
• You have added the rule to the rules() function in wdl-lint/src/lib.rs.
• You have added a test cases in wdl-lint/tests/lints that covers every
  possible diagnostic emitted for the rule within the file where the rule
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[peterhuene]

Looks good, just a few feedback comments to resolve.

[peterhuene]

Looks great!