feat(assert/unstable): add assertNever

[magurotuna]

This commit adds a new function assertNever to the assert submodule.

This function is particularly useful when we want to check whether we properly handle all the variants of a discriminated union.
Ref: https://www.typescriptlang.org/docs/handbook/unions-and-intersections.html#union-exhaustiveness-checking

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.25%. Comparing base (43f2a2d) to head (4fcf56e).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5690      +/-   ##
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- Coverage   96.25%   96.25%   -0.01%     
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  Files         476      477       +1     
  Lines       38501    38513      +12     
  Branches     5588     5589       +1     
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+ Hits        37060    37070      +10     
- Misses       1399     1401       +2     
  Partials       42       42              

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[kt3k]

We now have a policy of accepting all new features as unstable API. Could you add **UNSTABLE**: New API, yet to be vetted. note at the beginning of jsdoc and also add @experimental jsdoc tag? https://github.com/denoland/std/blob/010784cd03d0b3ac7e913d82d3468d293adc13c8/.github/CONTRIBUTING.md#notices-for-unstable-apis

[magurotuna]

done 👍 c12553a

[kt3k]

LGTM

[magurotuna]

@kt3k
Minor question, should we fix the instruction I referred to so we'll place <description> after the UNSTABLE warning and remove [!WARNING]?

[kt3k]

I think so, but @iuioiua might have a different idea. ref #5672 . We probably need discussion before updating it

[magurotuna]

Got it. I'll watch that issue 👍

[iuioiua]

This is the same as unreachable().

[magurotuna]

@iuioiua I thought that way, but actually unreachable does not do any compile-time checking while assertNever does.

This playground illustrates this:
https://www.typescriptlang.org/play/?#code/GYVwdgxgLglg9mABAQwM6oKYCcoDkMBu2AFAB4BciYh2AlJdUVogN4BQiiUAFlnAO5UMggKJY+WYgCJG2KbQDcbAL5s2oSLASJwWDMgjdkAIwA2GYgFtUAc0qooWGGBv0hTVhy68BQ0eLhJAANdfUMTc0QAEhZrG2UgxRU1KABPAAcMRAAVZBtEAF5EKQBBKUQAH2KAISkldXBoeCQQTBL0bDwaSSg8ylzXT05UfhgoQ0RiXsH2Tk4INCzSqXIvOcQAeg3EABM4RFQ4SwweZxs1ueM9ZABrJXWdjGBkEFMoVfXOLcRgQJs4KBcfZGMA7SJSWoAOhy3BgqEQelQryg8OcKC4GSy2AkkIunDQmBw+CYUzySU4qlUDU0zR0mAAqmBruEzBZpv0yUMDqNxtxJtNaFz5otimUPp9vnsDkcTrCXBdHs9keL1t9flh-oCoMDkKDwVCYXDdnAMPCIEd0jBzAAaFCg7x8fjw5CArDgWDHXGfHRMsJGVnSOBwdKoeT3RCUoA