url filter: start regex with word boundary

This allows recognizing more kinds of enclosed URIs, e.g:

 - angle brackets: <https://example.com>
 - lenticular brackets: 【https://example.com】
 - braces: {https://example.com}
 - guillemets: «https://example.com»
 - single guillemets ‹https://example.com›
 - ...

and also URIs right after an equals sign, e.g: foo=https://example.com.

BUG: 462511
BUG: 455166