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>

runtime/lua/nvim/tutor.lua

@@ -0,0 +1,80 @@
+---@class nvim.TutorMetadata
+---@field expect table<string, string|-1>
+
+---@alias nvim.TutorExtmarks table<string, string>
+
+---@type nvim.TutorExtmarks?
+vim.b.tutor_extmarks = vim.b.tutor_extmarks
+
+---@type nvim.TutorMetadata?
+vim.b.tutor_metadata = vim.b.tutor_metadata
+
+local sign_text_correct = '✓'
+local sign_text_incorrect = '✗'
+local tutor_mark_ns = vim.api.nvim_create_namespace('nvim.tutor.mark')
+local tutor_hl_ns = vim.api.nvim_create_namespace('nvim.tutor.hl')
+
+local M = {}
+
+---@param line integer 1-based
+local function check_line(line)
+  if vim.b.tutor_metadata and vim.b.tutor_metadata.expect and vim.b.tutor_extmarks then
+    local ctext = vim.fn.getline(line)
+
+    local extmarks = vim.api.nvim_buf_get_extmarks(
+      0,
+      tutor_mark_ns,
+      { line - 1, 0 },
+      { line - 1, -1 }, -- the extmark can move to col > 0 if users insert text there
+      {}
+    )
+    for _, extmark in ipairs(extmarks) do
+      local mark_id = extmark[1]
+      local expct = vim.b.tutor_extmarks[tostring(mark_id)]
+      local expect = vim.b.tutor_metadata.expect[expct]
+      local is_correct = expect == -1 or ctext == expect
+
+      vim.api.nvim_buf_set_extmark(0, tutor_mark_ns, line - 1, 0, {
+        id = mark_id,
+        sign_text = is_correct and sign_text_correct or sign_text_incorrect,
+        sign_hl_group = is_correct and 'tutorOK' or 'tutorX',
+        -- This may be a hack. By default, all extmarks only move forward, so a line cannot contain
+        -- any extmarks that were originally created for later lines.
+        priority = tonumber(expct),
+      })
+    end
+  end
+end
+
+function M.apply_marks()
+  vim.cmd [[hi! link tutorExpect Special]]
+  if vim.b.tutor_metadata and vim.b.tutor_metadata.expect then
+    vim.b.tutor_extmarks = {}
+    for expct, _ in pairs(vim.b.tutor_metadata.expect) do
+      ---@diagnostic disable-next-line: assign-type-mismatch
+      local lnum = tonumber(expct) ---@type integer
+      vim.api.nvim_buf_set_extmark(0, tutor_hl_ns, lnum - 1, 0, {
+        line_hl_group = 'tutorExpect',
+      })
+
+      local mark_id = vim.api.nvim_buf_set_extmark(0, tutor_mark_ns, lnum - 1, 0, {})
+
+      -- Cannot edit field of a Vimscript dictionary from Lua directly, see `:h lua-vim-variables`
+      ---@type nvim.TutorExtmarks
+      local tutor_extmarks = vim.b.tutor_extmarks
+      tutor_extmarks[tostring(mark_id)] = expct
+      vim.b.tutor_extmarks = tutor_extmarks
+
+      check_line(lnum)
+    end
+  end
+end
+
+function M.apply_marks_on_changed()
+  if vim.b.tutor_metadata and vim.b.tutor_metadata.expect and vim.b.tutor_extmarks then
+    local lnum = vim.fn.line('.')
+    check_line(lnum)
+  end
+end
+
+return M