feat: section ordering lint rule

[adthrasher]

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

• Rule Name: SectionOrdering

This rule ensures that tasks and workflows have their sections in the desired order.

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Rule specific checks:

• 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
  is implemented.
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[claymcleod]

I'm good with this. I'd take a look at my question about which span to report and also removing the println!. One last thing to consider is whether we want to report in the fix only the sections that were actually detected rather than a static list of all possible sections. I don't think that's required, but it might be just a tad bit cleaner—if nothing else, for future work.

[claymcleod]

Ah, also need to add this when #110 is merged in. You probably want to rebase on that branch, as doing the inverse would be difficult.

    fn document(&mut self, _: &mut Self::State, reason: VisitReason, _: &Document) {
        if reason == VisitReason::Exit {
            return;
        }

        // Reset the visitor upon document entry
        *self = Default::default();
    }

[peterhuene]

I'm happy to also rebase my branch if we want to merge this first.

[peterhuene]

This looks really good, just a few comments for your review.

[peterhuene]

Looks good!