feat: use extmarks instead of match

[Ramilito]

Investigating if this is a good idea for highlighting and maybe for keeping track of rows for acting on them.

• no need for concealed symbols
• possibility to add more utility to it in the future (for example virtual text for sorting visualisation)

• slower, we have to iterate through each row to assign the extmarks (testing shows that this is about 2ms for 2k marks)
• not as easy to determine where to highlight, need to calculate row/columns
• seems a bit taxing on neovim (was caused by not clearing the namespace)