refactor(tutor): reimplement interactive marks as extmark in Lua

Problem:
From https://matrix.to/#/!cylwlNXSwagQmZSkzs:matrix.org/$Ofj-TFIsEMbp0O9OhE8xuZSNi-nhRLtZTOgs6JRLNrs?via=matrix.org&via=gitter.im&via=mozilla.org

In lesson 2.6, users are asked to remove the second, forth and fifth
lines with `dd` command, then they are asked to undo twice to make the
text go back to original state. But after that, the mark ✗ appears
again, which confuses the user because they think they do something
wrong. This is a limitation with the current implementation, which is
based on line number only.

Solution:
Reimplement interactive marks as extmarks in Lua. This also make the
feature less fragile, as users can remove, add some arbitrary lines
without breaking the interactive marks.

Co-authored-by: Justin M. Keyes <justinkz@gmail.com>

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