(refactor)org-roam-unlinked-references-section: make steps advisable

[okomestudio]

Motivation for this change

This PR implements a proposed refactoring of the org-roam-unlinked-references-section function, so that

• The word boundary regex is supplied via the custom variable org-roam-unlinked-references-word-boundary-re to allow more complex logic
• The functions org-roam-unlinked-references-title-regex, org-roam-unlinked-reference-apply-word-boundary-re, org-roam-unlinked-references-file-glob-args, and org-roam-unlinked-references-result-filter-p are refactored out to allow targeted advising for custom behavior
• The org-roam-unlinked-references-max-results-count custom variable limits result to avoid “freezing” when too many results are matched
• The inner loop rendering the preview line is passed the values for file, row number, and col number from the previous iteration to allow more controlled rendering, in case when the matched term appears multiple times within the same line

The overall behavior of the refactored function does not change. The rationale for this change is described in #2416.