refactor: require bibtex package in citar.el

This ensures that the `bibtex-beginning-of-entry',
`bibtex-search-forward-field', and `bibtex-set-dialect' functions in citar.el
are loaded before attempting to byte-compile the file.

This should resolve the issue in
#872 (comment).

Author: arifer612

citar.el

@@ -26,6 +26,7 @@
   (require 'subr-x))
 (require 'seq)
 (require 'map)
+(require 'bibtex)
 (require 'browse-url)
 (require 'citar-cache)
 (require 'citar-format)