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.