fix: reworks the concept of lint directives

[a-frantz]

Describe the problem or feature in addition to a link to the issues.

This PR reworks the concept of lint directives (AKA #@ comments or except comments). It should close #135 , which details how lint directives have been partially broken since their introduction.

Lint directives keep their same form (#@ except: <comma delimited list of Rule IDs>) but now are only expected in certain locations. Those locations are defined on a per-rule basis via the new exceptable_nodes() method in the Rule trait.

TODO in follow-up PR[s]

• We've discussed the idea of firing diagnostics for lint directives that have no effect (but are in the correct location). This will require some further changes to the Diagnostics struct not included here.
• IMO the WDL in our tests/lints/ dir is awfully messy. They frequently contain a long list of excepted rules. All those WDL files deserve a clean up to be more conformant to expected lints.
• Refactor PreambleWhitespace and PreambleComments into new rules PreambleFormatting and PreambleCommentInBody (accepting alternate names there). PreambleFormatting will perform the bulk of the checks currently handled by the whitespace and comments rules. PreambleCommentInBody will check for double-pound-sign comments outside the preamble.

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[peterhuene]

This looks really great! I've added some review comments.

[peterhuene]

Looks great!