Fix .gitignore to allow new `+types/+untyped` and `+types/+util` functions

[lawrence-mbf]

We currently use the negation operator to negate all generated class files from being added to the repository. Unfortunately, due to a quirk in git, this negation doesn't completely work when adding new files even in "negated" directories: https://www.atlassian.com/git/tutorials/saving-changes/gitignore

The best solution right now is to add common gitignore records for core, hdmf-common, and hdmf-experimental and practice due diligence in removing extension schemas in subsequent PRs.

[lawrence-mbf]

One other workaround is for developers to remember to use git add -f when adding new files.

[lawrence-mbf]

Alternatively, this might not be a huge issue since most developers won't be adding +types/+untyped and +types/+util functions.