Border Fantasy

This PR adds support for fancier borders, fixes some bugs, and adds a helper function for measuring text blocks.

With the new border support, it's possible to construct borders from multiple runes, such as this:

New

• Support for multi-rune borders
• Added Size() (int, int) for returning the width and the height of a block in one go

Fixed

• Bump go-runewidth to fix some emoji mis-measuring
• Fix a panic that could happen in custom borders

Thoughts? Questions? We love hearing from you. Feel free to reach out on Twitter, The Fediverse or right here in GitHub Discussions.