range: add extmark utilities

spec/range_spec.lua

@@ -617,4 +617,29 @@ describe('Range', function()
       }, vim.api.nvim_buf_get_lines(b, 0, -1, false))
     end)
   end)
+
+  it('can save to extmark', function()
+    withbuf({
+      'The quick brown',
+      'fox',
+      'jumps',
+      'over',
+      'the lazy dog',
+    }, function()
+      -- Construct a range over 'fox jumps'
+      local r = Range.new(Pos.new(nil, 2, 1), Pos.new(nil, 3, Pos.MAX_COL), 'V')
+      local extrange = r:save_to_extmark()
+      assert.are.same({ 'fox', 'jumps' }, extrange:range():lines())
+      -- change 'jumps' to 'leaps':
+      vim.api.nvim_buf_set_text(extrange.bufnr, 2, 0, 2, 4, { 'leap' })
+      assert.are.same({
+        'The quick brown',
+        'fox',
+        'leaps',
+        'over',
+        'the lazy dog',
+      }, vim.api.nvim_buf_get_lines(extrange.bufnr, 0, -1, false))
+      assert.are.same({ 'fox', 'leaps' }, extrange:range():lines())
+    end)
+  end)
 end)