Support multiple, nested aliases of fields in REST exports

Thanks @ericpoir! I was able to fix the regular expression so the alias you mention in #9 works correctly. I'm reverting your changes so we can just rely on the regexp for all cases.

Other improvements:

• Added detection of alias conflicts: e.g. 'a.b' conflicts with 'a.b.c'.
• Allowed to use '@' in an alias.
• Fixed bug where 'a b' would be accepted as an alias.
• Relax requirement for multiple aliases: both ", " and "," separators will be properly handled.

#12 also applies on top of Drupal 9.5.3.

This doesn't seem to be working on the site i'm currently working on which uses Drupal 9.5.9.

I'm getting the following error for each fields that contains dots:

"The machine-readable name must contain only letters, numbers, dashes and underscores."

Not sure why?

After upgrading our site to Drupal 10, I was getting warning saying that ${var} is now deprecated so I created the following patch to fix that.

Just run a quick diff of #12 vs #16; verified that the only change was replacing e.g. "${abc}" into "{$abc}".