[ENH] Do explicitly "allow" for having dotfiles and explicitly ignore them

[yarikoptic]

Up to this point it was up for a validator to "code" that up. E.g. current new deno based validator in
https://github.com/bids-standard/bids-validator/blob/main/src/files/ignore.ts has

const defaultIgnores = [
  '.git**',
  '.*',
  'sourcedata/',
  ...

thus ignores all dotfiles from validation. But this is inconsistent e.g. with shipped in bids-specification schematools treatment, where files in a dotdirectory were reported as "invalid":

❯ cat /tmp/input.txt | python -m bidsschematools pre-receive-hook
.datalad/config
.gitattributes
sub-01/anat/sub-01_unknown.nii.gz

And as demonstrated above there could be legit "system" dotfiles (VCS related etc) which are part of the folder containing BIDS dataset, I think we should explicitly describe position of BIDS in relation to the dotfiles. As it is pretty much "ignore", that is what this PR is intended to state.

In this PR I also adjusted pre_receive_hook to ignore dotfiles. The find_files was already ignoring them.

[effigies]

I think this is reasonable.

[effigies]

Do we want to consider a schema addition, or leave it as an instruction to implementations?

[effigies]

Merging. Can consider schema later.