refactor(lua): rewrite vim.highlight.range()

[zeertzjq]

• Use getregionpos().
• Use a single extmark for non-blockwise selection.

[clason]

This introduced a regression with diagnostics: if diagnostics are at the end of the line, the underline for it now extends to the beginning of the next line. I haven't cooked up a minimal example yet, but you can observe this in the lazy.nvim update output:

I think there still needs to be some distinction between single- and multiline extmarks (regarding the hl_eol option).

[zeertzjq]

I don't end of line or hl_eol are relevant. I think #29022 should fix this.

[clason]

Indeed it does, thank you!