chore(fs): document the deprecation of exists and existsSync

[iuioiua]

exists was deprecated because it causes a race condition when used before a file operation. However, it still has some legitimate uses cases, even within std. The community has expressed more arguments here:

• Deprecating "exists" because of some use cases ignores legitimate use cases #2594
• #2102

This PR:

1. Reverts the deprecation of exists and existsSync.
2. Advises against using exists before a file operation and clearly explains the reasoning.
3. Closes docs: give more detailed description/reasoning/alternative suggestions for the deprecation of fs.exists #2125.
4. Cleans up an illegitimate use of exists within the codebase.
5. Adds URL support for the argument.

A few legitimate use cases were also likely removed in #2546, as Deno.lstat is still used to check for the existence of files. It'd be a good idea to revisit these removals and consider re-adding the exists, but only where appropriate.

[kt3k]

The update of the document looks good to me. Also the removal of the usage in log is very nice!

But let's keep them deprecated for now because the core team is still in favor of deprecation.

Also in my view, the community is also not totally on the same page on reverting deprecation. Some people still prefer the deprecation, and there are suggestion of modified version of API instead of exists/existsSync. See the discussion in https://github.com/denoland/deno_std/discussions/2102 for details. I think we need more discussion before making decision on this

[iuioiua]

Ok, I can reduce the scope of this PR by ignoring the actual deprecation of it for now.

Please check my previous comment about the use of exists within this repo in std/log. How do we cater for that use case without using exists?

[kt3k]

Thanks for updating!

However, it still has some legitimate uses cases, even within std.

Ok. This looks more legitimate usage than typical ones. I also observe several usages in our testing like assertEquals(existsSync(srcDir), true);. I'll bring this up in core team meetings.

[kt3k]

LGTM