Do not set height classes in ModalDialog when size is 'custom'

[acelaya]

Part of hypothesis/client#6151 and hypothesis/client#6158

Depends on #1442

All ModalDialog instances have currently these hardcoded classes:

• max-w-[90vw] max-h-[90vh] -> Ensure they don't span more than 90% of the viewport's width and height.
• tall:fixed tall:max-h-[80vh] tall:top-[10vh] -> When the screen is tall ((min-height: 32rem)), ensure a fixed position of the modal.

This usually makes sense, as these play nicely with the regular sizes. However, when providing custom size, the classes which affect vertical size and positioning can get in the way and force you to override them with other classes including the important modifier (!).

This PR changes that behavior to make those classes be skipped when size is custom, so that consumer code has more control over how it behaves.

This is technically speaking a breaking change, but we are currently not using custom size modals in downstream projects.

The main motivation for this change is being able to use ModalDialog for the client's notebook, and benefit from its accessibility afordances.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (97130ff) 100.00% compared to head (1e3f910) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #1443   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           60        60           
  Lines          934       936    +2     
  Branches       351       353    +2     
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+ Hits           934       936    +2     

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