llmsPostsSelect2 improvements

[brianhogg]

Description

Allowed llmsPostsSelect2 to be called on a group with different data-post-type and other attributes. Also adds data-placeholder support.

Fixes #2805

How has this been tested?

Manually

Checklist:

• [x] This PR requires and contains at least one changelog file. <!-- To create a changelog yml file: npm run dev changelog add -- -i and follow the prompt. See also: https://github.com/gocodebox/lifterlms/blob/trunk/packages/dev/README.md#changelog-add -->
• [x] My code has been tested.
• [ ] My code passes all existing automated tests.
• [x] My code follows the LifterLMS Coding & Documentation Standards.