Fix holdReset glitches for async resets and add registerSyncReset (copy #3116)

[mergify]

Aimed to address #3115

This updates the documentation of holdReset to indicate the problems it has. It also gives a new function stretchReset that should have the correct behaviour but has slightly different timing characteristics so it's not a drop in replacement.

I have marked the holdReset function with a warning, I don't like it since now we introduce warning into our own project when exposing it and using it in tests. I also thought a deprecation pragma. Both have some implication as that requires a version bump according to haskell PVP specification.

Still TODO:

• Write a changelog entry (see changelog/README.md)
• Check copyright notices are up to date in edited files
• Write test
• Adjust doctest

---

This is an automatic copy of pull request #3116 done by [Mergify](https://mergify.com).

[mergify]

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Changes to be committed:
	new file:   changelog/2026-01-08T16_58_32+01_00_add_registerSyncReset
	new file:   changelog/2026-01-08T16_58_32+01_00_fix_holdreset
	modified:   clash-prelude/src/Clash/Explicit/Reset.hs
	modified:   tests/Main.hs
	new file:   tests/shouldwork/Signal/HoldResetAsync.hs
	new file:   tests/shouldwork/Signal/HoldResetSync.hs

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   clash-prelude/src/Clash/Signal.hs
	both modified:   tests/shouldwork/Signal/ResetGen.hs

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