fix: Correct header highlighting for multi-byte characters

[ctchen222]

The previous implementation used vim.fn.strwidth() to calculate the end column for header highlighting. This function returns the display width, which can differ from the byte length when multi-byte characters like emojis are present.

Since the end_col for extmarks expects a byte-based index, this caused the highlighting to be applied incorrectly.

This patch corrects the issue by using the byte length (#header_value) for the end_col of the highlight extmark, while still using vim.fn.strwidth() for virt_text_win_col to ensure correct visual alignment of UI elements.

Fixes #1384

[ctchen222]

Fix #1384

[deathbeam]

thanks!

@all-contributors add @ctchen222 for code

[allcontributors]

@deathbeam

I've put up a pull request to add @ctchen222! 🎉