refactor: establish ViewTransitions folder feature

[jderochervlk]

Tracking issue: #342

Stack context

Part 6 of the Option 5 folder-feature stack.

• Previous: refactor: establish Performance folder feature #315
• Next: another dependency-clean domain folder

Review this PR as the incremental diff against #315.

Summary

• rename src/ViewTransitions to the target src/view-transitions folder
• change the folder to the unqualified internal ViewTransitions feature
• add the supported WebAPI.ViewTransitions feature with its WebAPI.Event dependency
• preserve the flat public WebAPI.ViewTransition module

The internal ViewTransitionsTypes helper stays private inside the complete folder.

Temporary state

• this PR does not introduce compatibility aliases, file exclusions, or module slices; its folder and feature ownership are final
• helper-type colocation remains deferred to the final cleanup layer

Review focus

• the WebAPI.ViewTransitions -> WebAPI.Event + ViewTransitions implication
• unchanged public module path and binding signatures
• complete folder ownership

Verification

• node_modules/.bin/rescript clean
• node_modules/.bin/rescript build --prod --features DOM,Event,ViewTransitions
• npm run build
• npm test
• npm run format:check
• git diff --check

[jderochervlk]

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