new link ids

[willmcgugan]

Adds a link id to the Style class. Link ids are used when generating the ansi codes for links, so that the terminal knows which cells to highlight.

Formerly if a link contained multiple styles, the terminal would highlight individually styled portions of text on hover rather than the entire link.

For example, printing this would look like the words Hello and World were two links, when they are actually one link.

"[link=https://example.org]Hello [b]World![/b][/link]"

This PR fixes that.

Type of changes

• Bug fix
• New feature
• Documentation / docstrings
• Tests
• Other

Checklist

• I've run the latest black with default args on new code.
• I've updated CHANGELOG.md and CONTRIBUTORS.md where appropriate.
• I've added tests for new code.
• I accept that @willmcgugan may be pedantic in the code review.