Allow word boundary regex to be customized for unlinked reference

[okomestudio]

Motivation for this change

The unlinked references are searched for by ripgrep using a simple regex pattern (\b%s\b). This works fine with English, but in a language for which the concept of regex word boundary doesn't apply (e.g., CJK), the pattern doesn't work. The issue #1290 for Chinese is an example.

This PR introduces org-roam-unlinked-references-word-boundary-re custom variable so that users can modify the regex pattern used by ripgrep.

For example, adding a pattern like this

(set-q org-roam-unlinked-references-word-boundary-re
       "|(\\b%1$s\\b|(?<=[^\x20-\x7e\xff61-\xff9f])%1$s(?=[^\x20-\x7e\xff61-\xff9f]))")

the regex would work sufficiently well in both English and Japanese.

This PR should fix #1290, though improving regex pattern is delegated to users.

[okomestudio]

To avoid clutter, closing this PR in favor of #2416.