Restore viewport behaviour

[tmeasday]

Issue: #5757 and #5818

A few things were going on here:

• You no longer got a set of viewports by default by just registering the addon
• You couldn't set the defaultViewport any more

What I did

(Partially) restored the old API (still via the parameter API), added warnings for no-longer supported stuff, and documented it.

NOTE: we lost the onViewportChange option. I'm not sure that was very widely used? Maybe we'll hear about it if so 🤷‍♂️

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[codecov]

Codecov Report

Merging #5829 into next will decrease coverage by 0.04%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##             next    #5829      +/-   ##
==========================================
- Coverage   34.28%   34.24%   -0.05%     
==========================================
  Files         651      651              
  Lines        9473     9482       +9     
  Branches     1344     1374      +30     
==========================================
- Hits         3248     3247       -1     
- Misses       5611     5617       +6     
- Partials      614      618       +4

Impacted Files	Coverage Δ
addons/viewport/preview.js	100% <ø> (ø)	⬆️
addons/viewport/src/legacy_preview/withViewport.js	0% <ø> (ø)	⬆️
addons/viewport/src/Tool.js	0% <0%> (ø)	⬆️
addons/viewport/src/legacy_preview/index.js	0% <0%> (ø)	⬆️
addons/viewport/src/constants.js	0% <0%> (ø)	⬆️

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[shilman]

Thanks @tmeasday, this is awesome!